Reducing Hydrogen Iodide Content in Carbonylation Processes

ABSTRACT

Processes for producing acetic acid wherein at least one process vent stream is treated in an absorber column that utilizes a scrubber solvent, e.g., acetic acid, methanol, and/or methyl acetate to remove hydrogen iodide therefrom.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a continuation-in-part of U.S.Non-Provisional patent application Ser. No. 14/694,883, filed Apr. 23,2015, which claims priority to U.S. Provisional Patent Application No.62/079,961, filed Nov. 14, 2014, the disclosure of these applicationsare incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, inparticular, to improved processes for producing acetic acid thateffectively reduce hydrogen iodide content in various process streams.

BACKGROUND OF THE INVENTION

A widely used and successful commercial process for synthesizing aceticacid involves the catalyzed carbonylation of methanol, e.g., a methanol(feed) composition, with carbon monoxide. The catalyst may containrhodium and/or iridium and a halogen promoter, typically methyl iodide.The reaction is conducted by continuously bubbling carbon monoxidethrough a liquid reaction medium in which the catalyst is dissolved. Thereaction medium comprises acetic acid, methyl acetate, water, methyliodide and the catalyst. The methanol and the carbon monoxide come intocontact in the reaction medium and react with one another to form crudeacetic acid. Conventional commercial processes for the carbonylation ofmethanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259,5,026,908, and 5,144,068, the entireties of which are incorporatedherein by reference. Another conventional methanol carbonylation processincludes the Cativa™ process, which is discussed in Jones, J. H. (2002),“The Cativa™ Process for the Manufacture of Acetic Acid,” PlatinumMetals Review, 44 (3): 94-105, the entirety of which is incorporatedherein by reference.

The carbonylation reaction forms reaction by-products such as water,(residual) methyl iodide, and carbonyl impurities, e.g., acetaldehyde.These by-products are typically separated and/or recycled within thesystem. Hydrogen iodide is another undesirable component that may bepresent in the reaction medium. The presence of hydrogen iodide in thereaction system is particularly troublesome because it acts as acorrosion agent that may cause metallurgical problems throughout thereaction and separation zones. Thus, the need exists for reducing theformation of hydrogen iodide in carbonylation processes.

US Patent Application Publication No. 2013/0116470 discloses aproduction process of acetic acid comprising a reaction step forcontinuously allowing at least one member selected from the groupconsisting of methanol, dimethyl ether, and methyl acetate to react withcarbon monoxide in a catalyst system comprising a rhodium catalyst, aniodide salt, and methyl iodide in the presence of acetic acid and waterin a plant compromising a reactor; a flasher; and a distillation column;wherein part of the vaporized stream is introduced into a heatexchanger. The process achieves a production of acetic acid with a highpurity in a resource-saving and energy-saving equipment by efficientlyremoving a reaction heat even in a large-sized plant.

U.S. Pat. No. 8,318,977 discloses a methanol carbonylation systemincluding an absorber tower adapted for receiving a vent gas stream andremoving methyl iodide therefrom with a scrubber solvent, the absorbertower being coupled to first and second extractant sources, which arecapable of supplying different first and second extractants. A switchingsystem including valves alternatively provides first or secondextractants to the absorber tower and returns the used solvent andabsorbed material to the carbonylation system to accommodate differentoperating modes. This reference does not mention hydrogen iodide.

While the above-described processes provide general processes forpurifying a crude acetic acid product, these processes fail tospecifically address the separation of hydrogen iodide from the crudeproduct. The need exists for improved processes for producing aceticacid that provide for separation and removal of hydrogen iodide from acrude acetic acid product.

SUMMARY OF THE INVENTION

This invention relates to processes producing acetic acid, the processescomprising the steps of carbonylating, in a reactor, at least one ofmethanol, dimethyl ether, and methyl acetate in a reaction mediumcomprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and a finite amount water, to form a crude aceticacid product comprising acetic acid and optionally venting from thereactor a reactor vent stream comprising hydrogen iodide and directingthe reactor vent stream to the absorber tower. The processes furthercomprise the steps of flashing the crude acetic acid product, with orwithout heat, to form a first vapor stream comprising acetic acid andmethyl iodide (and optionally hydrogen iodide) and a first liquidresidue stream comprising metal catalyst and halide salt and optionallycondensing at least a portion of the first vapor stream to form aflasher vent stream comprising hydrogen iodide and directing the flashervent stream to the absorber tower. The process further comprise the stepof separating, in a light ends column, the first vapor stream to form asecond vapor stream comprising methyl iodide and hydrogen iodide, a sidedraw comprising purified acetic acid product, and a second liquidresidue stream. The second vapor stream may be decanted to form adecanter vent stream comprising hydrogen iodide and the at least aportion of the second vapor stream may comprise the decanter ventstream. The separating further may further comprise condensing at leasta portion of the second vapor stream to form a light ends vent streamcomprising hydrogen iodide and directing the light ends vent stream toan absorber tower. The processes further comprise the steps of feedingto the absorber tower at least a portion of at least one of the reactorvent stream, the flasher vent stream, and the second vapor stream;contacting the absorber tower feed with a first absorbent comprisingacetic acid to absorb methyl iodide and to form a first absorber returnstream comprising the first absorbent and the absorbed methyl iodide;conveying the first absorber return stream to the light ends columnand/or drying column; and terminating the supply of first absorbent tothe absorber tower. The processes further comprise the steps ofcontacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second absorber return stream comprising the secondabsorbent and the absorbed methyl iodide; and conveying the secondabsorber return stream to the reactor. The process may further comprisethe step of chilling the first absorbent and/or the second absorbentprior to the respective contacting. The process may further comprise thestep of mixing the second absorber return stream with methanol or areactive derivative thereof prior to the conveying. In one embodiment,the absorber tower feed comprises methyl acetate and the first andsecond absorbents absorb methyl acetate from the absorber tower feed. Inone embodiment, the process does not employ a stripper column to treatthe first absorbent and/or the second absorbent. The carbonylating maybe conducted while maintaining the reaction system at concentrations of2 to 25% by weight iodide salt, 1 to 20% by weight methyl iodide, 0.1 to30% by weight methyl acetate, and 0.1 to 10% by weight water. The secondabsorber return stream may not be returned to the separation train. Thecontacting of the absorber tower feed with the methanol or methylacetate in the second absorbent may form methyl iodide. The conveying ofthe second absorber return stream may comprise the steps of conveyingthe second absorber return stream to the reactor during a transitionperiod; following the transition period, continuing to feed the secondabsorber return stream to the reactor; and conveying the second absorberreturn stream to the reactor after the termination of the supply offirst absorbent. The second vapor stream further comprises volatilecomponents.

The invention also relates to processes for operating an absorber towerin a carbonylation process comprising the steps of carbonylating, in areactor, at least one of methanol, dimethyl ether, and methyl acetate ina reaction medium comprising a metal catalyst, methyl iodide, an iodidesalt, and optionally acetic acid and a finite amount water, to form acrude acetic acid product comprising acetic acid and optionally ventingfrom the reactor a reactor vent stream comprising hydrogen iodide anddirecting the reactor vent stream to the absorber tower. The processesfurther comprise the steps of flashing the crude acetic acid product,with or without heat, to form a first vapor stream comprising aceticacid and methyl iodide (and optionally hydrogen iodide) and a firstliquid residue stream comprising metal catalyst and halide salt andoptionally condensing at least a portion of the first vapor stream toform a flasher vent stream comprising hydrogen iodide and directing theflasher vent stream to the absorber tower. The process further comprisethe step of separating, in a light ends column, the first vapor streamto form a second vapor stream comprising methyl iodide and hydrogeniodide, a side draw comprising purified acetic acid product, and asecond liquid residue stream. The second vapor stream may be decanted toform a decanter vent stream comprising hydrogen iodide and the at leasta portion of the second vapor stream may comprise the decanter ventstream. The separating further may further comprise condensing at leasta portion of the second vapor stream to form a light ends vent streamcomprising hydrogen iodide and directing the light ends vent stream toan absorber tower. The process may further comprise the step of feedingto an absorber tower at least a portion of at least one of the reactorvent stream, the flasher vent stream, and the second vapor stream toinitiate a start-up period and, during the start-up period, contactingthe absorber tower feed with a first absorbent comprising acetic acid toabsorb methyl iodide and to form a first absorber return streamcomprising the first absorbent and the absorbed methyl iodide; conveyingthe first absorber return stream to the light ends column and/or dryingcolumn; transitioning from start up to steady operation during achangeover period by terminating the supply of first absorbent to theabsorber tower. The process further comprises the step of providing tothe absorber tower a second absorbent comprising methanol and/or methylacetate, wherein, during at least a portion of the changeover period,acetic acid, methanol and methyl iodide are provided to the absorbertower; forming a combined absorber return stream comprising acetic acid,methanol, and methyl iodide; conveying the combined absorber returnstream from the absorber tower to the reactor; initiating steady stateoperation after the changeover period, and, during steady stateoperation, contacting the second vapor stream with the second absorbentto absorb methyl iodide, hydrogen iodide to form a second absorberreturn stream comprising the second absorbent and the absorbed methyliodide. The processes further comprise the steps of during the steadystate operation, mixing the second absorber return stream with methanolor a reactive derivative thereof to form a mixed stream; and conveyingthe mixed stream to the reactor. The processes may further comprise thestep of chilling the first absorbent and/or the second absorbent priorto the respective contacting. The termination of supply of firstabsorbent to the absorber tower and provision of the second absorbent tothe absorber tower may occur substantially simultaneously.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide; condensing at least a portion of the reactorvent stream to form a condensed reactor vent stream; feeding to anabsorber tower an absorber tower feed comprising at least a portion ofthe condensed reactor vent stream; contacting the absorber tower feedwith a first absorbent selected from the group consisting of aceticacid, methanol, and methyl acetate, preferably acetic acid, to absorbmethyl iodide and hydrogen iodide and to form a first extract comprisingthe first absorbent and the absorbed methyl iodide and hydrogen iodide;conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column; decreasing the supply of first absorbent tothe absorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; feeding to an absorber tower an absorber tower feedcomprising at least a portion of the flasher vent stream (or thecondensed flasher vent stream); contacting the absorber tower feed witha first absorbent selected from the group consisting of acetic acid,methanol, and methyl acetate, preferably acetic acid, to absorb methyliodide and hydrogen iodide and to form a first extract comprising thefirst absorbent and the absorbed methyl iodide and hydrogen iodide;conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column; decreasing the supply of first absorbent tothe absorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt;separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream; optionally condensing the second vapor stream, feedingto an absorber tower an absorber tower feed comprising at least aportion of the second vapor stream; contacting the absorber tower feedwith a first absorbent selected from the group consisting of aceticacid, methanol, and methyl acetate, preferably acetic acid, to absorbmethyl iodide and hydrogen iodide and to form a first extract comprisingthe first absorbent and the absorbed methyl iodide and hydrogen iodide;conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column; decreasing the supply of first absorbent tothe absorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor. Two absorbent steps areemployed in this embodiment.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream; decanting the second vapor stream to form a light phasecomprising acetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;separating at least a portion of the light phase and/or the heavy phaseto a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiver;optionally venting from the overhead receiver a PRS overhead receivervent stream comprising hydrogen iodide; feeding to an absorber tower anabsorber tower feed comprising at least a portion of the PRS overheadreceiver vent stream and/or the PRS overhead; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate, preferably acetic acid, toabsorb methyl iodide and hydrogen iodide to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; separating, in a light ends column, the flashed firstvapor stream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a first column and asecond column to obtain a second PRS overhead comprising hydrogen iodidefrom the second PRC removal column; optionally collecting the second PRSoverhead in an overhead receiver and venting from the overhead receivera second PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second PRS overhead receiver vent stream and/or thesecond PRS overhead; contacting the absorber tower feed with a firstabsorbent selected from the group consisting of acetic acid, methanol,and methyl acetate to absorb hydrogen iodide to form a first extractcomprising absorbed hydrogen iodide; and optionally contacting theabsorber tower feed with a second absorbent comprising methanol and/ormethyl acetate to absorb methyl iodide and hydrogen iodide and to form asecond extract comprising the second absorbent and the absorbed methyliodide.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; separating, in a light ends column, the flashed firstvapor stream (or the condensed first vapor stream) to form a secondvapor stream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; decanting the second vapor stream to form a light phasecomprising acetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;separating at least a portion of the light phase and/or the heavy phasein a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiverand optionally venting from the overhead receiver a PRS overheadreceiver vent stream comprising hydrogen iodide; feeding to an absorbertower an absorber tower feed comprising at least a portion of the PRSoverhead receiver vent stream and/or the PRS overhead; contacting theabsorber tower feed with a first absorbent selected from the groupconsisting of acetic acid, methanol, and methyl acetate, preferablyacetic acid, to absorb hydrogen iodide to form a first extractcomprising absorbed hydrogen iodide; and optionally conveying the secondextract, directly or indirectly, to the reactor.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; separating, in a light ends column, the flashed firstvapor stream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a single column toform a PRS overhead comprising hydrogen iodide; optionally collectingthe PRS overhead in an overhead receiver and venting from the overheadreceiver a PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead; contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate to absorb hydrogen iodide to form a first extract comprisingabsorbed hydrogen iodide; and optionally contacting the absorber towerfeed with a second absorbent comprising methanol and/or methyl acetateto absorb methyl iodide and hydrogen iodide and to form a second extractcomprising the second absorbent and the absorbed methyl iodide.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; feeding to an absorber tower an absorber tower feedcomprising at least a portion of the flasher vent stream (or thecondensed flasher vent stream); contacting the absorber tower feed witha first absorbent selected from the group consisting of acetic acid,methanol, and methyl acetate, preferably acetic acid, to absorb methyliodide and hydrogen iodide to form a first extract comprising the firstabsorbent, hydrogen iodide, and methyl iodide.

The invention also relates to a process comprising the steps of:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt;separating, in a light ends column, the flashed first vapor stream (orthe condensed flasher vent stream) to form a second vapor streamcomprising methyl iodide and hydrogen iodide, a side draw comprisingpurified acetic acid product, and a second liquid residue stream;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second vapor stream; contacting the absorber tower feedwith a first absorbent selected from the group consisting of aceticacid, methanol, and methyl acetate, preferably acetic acid, to absorbmethyl iodide and hydrogen iodide to form a first extract comprising thefirst absorbent, hydrogen iodide, and methyl iodide.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appendednon-limiting figures, wherein:

FIG. 1 shows a schematic of an acetic acid production process inaccordance with the present invention.

FIG. 2 shows a schematic of an absorber tower unit and the accompanyingcomponentry in accordance with the present invention.

FIG. 3 shows a schematic of an acetic acid production process inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Introduction

At the outset, it should be noted that in the development of any suchactual embodiment, numerous implementation—specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem related and business related constraints, which will vary fromone implementation to another. In addition, the processes disclosedherein can also comprise components other than those cited orspecifically referred to, as is apparent to one having average orreasonable skill in the art.

In the summary and this detailed description, each numerical valueshould be read once as modified by the term “about” (unless alreadyexpressly so modified), and then read again as not so modified unlessotherwise indicated in context. Also, in the summary and this detaileddescription, it should be understood that a concentration range listedor described as being useful, suitable, or the like, is intended thatany and every concentration within the range, including the end points,is to be considered as having been stated. For example, a range “from 1to 10” is to be read as indicating each and every possible number alongthe continuum between about 1 and about 10. Thus, even if specific datapoints within the range, or even no data points within the range, areexplicitly identified or refer to only a few specific data points, it isto be understood that inventors appreciate and understand that any andall data points within the range are to be considered to have beenspecified, and that inventors possessed knowledge of the entire rangeand all points within the range.

Throughout the entire specification, including the claims, the followingterms have the indicated meanings unless otherwise specified.

As used in the specification and claims, “near” is inclusive of “at.”The term “and/or” refers to both the inclusive “and” case and theexclusive “or” case, and is used herein for brevity. For example, amixture comprising acetic acid and/or methyl acetate may comprise aceticacid alone, methyl acetate alone, or both acetic acid and methylacetate.

All percentages are expressed as weight percent (wt. %), based on thetotal weight of the particular stream or composition present, unlessotherwise noted. Room temperature is 25° C. and atmospheric pressure is101.325 kPa unless otherwise noted.

For purposes herein:

acetic acid may be abbreviated as “AcOH”;

acetaldehyde may be abbreviated as “AcH”;

methyl acetate may be abbreviated “MeAc”;

methanol may be abbreviated “MeOH”;

methyl iodide may be abbreviated as “MeI”;

hydrogen iodide may be abbreviated as “HI”;

carbon monoxide may be abbreviated “CO”; and

dimethyl ether may be abbreviated “DME”.

HI refers to either molecular hydrogen iodide or dissociated hydriodicacid when at least partially ionized in a polar medium, typically amedium comprising at least some water. Unless otherwise specified, thetwo are referred to interchangeably. Unless otherwise specified, HIconcentration is determined via acid-base titration using apotentiometric end point. In particular, HI concentration is determinedvia titration with a standard lithium acetate solution to apotentiometric end point. It is to be understood that for purposesherein, the concentration of HI is not determined by subtracting aconcentration of iodide assumed to be associated with a measurement ofcorrosion metals or other non H+ cations from the total ionic iodidepresent in a sample.

It is to be understood that HI concentration does not refer to iodideion concentration. HI concentration specifically refers to HIconcentration as determined via potentiometric titration.

This subtraction method is an unreliable and imprecise method todetermine relatively lower HI concentrations (i.e., less than about 5weight percent) due to the fact that it assumes all non-H+ cations (suchas cations of Fe, Ni, Cr, Mo) are associated with iodide anionexclusively. In reality, a significant portion of the metal cations inthis process can be associated with acetate anion. Additionally, many ofthese metal cations have multiple valence states, which adds even moreunreliability to the assumption on the amount of iodide anion whichcould be associated with these metals. Ultimately, this method givesrise to an unreliable determination of the actual HI concentration,especially in view of the ability to perform a simple titration directlyrepresentative of the HI concentration.

For purposes herein, an “overhead” or “distillate” of a distillationcolumn refers to at least one of the lower boiling condensable fractionswhich exits at or near the top, (e.g., proximate to the top), of thedistillation column, and/or the condensed form of that stream orcomposition. Obviously, all fractions are ultimately condensable, yetfor purposes herein, a condensable fraction is condensable under theconditions present in the process as readily understood by one of skillin the art. Examples of noncondensable fractions may include nitrogen,hydrogen, and the like. Likewise, an overhead stream may be taken justbelow the upper most exit of a distillation column, for example, whereinthe lowest boiling fraction is a non-condensable stream or represents ade-minimis stream, as would be readily understood by one of reasonableskill in the art.

The “bottoms” or “residuum” of a distillation column refers to one ormore of the highest boiling fractions which exit at or near the bottomof the distillation column, also referred to herein as flowing from thebottom sump of the column. It is to be understood that a residuum may betaken from just above the very bottom exit of a distillation column, forexample, wherein the very bottom fraction produced by the column is asalt, an unusable tar, a solid waste product, or a de-minimis stream aswould be readily understood by one of reasonable skill in the art.

For purposes herein, distillation columns comprise a distillation zoneand a bottom sump zone. The distillation zone includes everything abovethe bottom sump zone, i.e., between the bottom sump zone and the top ofthe column. For purposes herein, the bottom sump zone refers to thelower portion of the distillation column in which a liquid reservoir ofthe higher boiling components is present (e.g., the bottom of adistillation column) from which the bottom or residuum stream flows uponexiting the column. The bottom sump zone may include reboilers, controlequipment, and the like.

It is to be understood that the term “passages”, “flow paths”, “flowconduits”, and the like in relation to internal components of adistillation column are used interchangeably to refer to holes, tubes,channels, slits, drains, and the like, which are disposed through and/orwhich provide a path for liquid and/or vapor to move from one side ofthe internal component to the other side of the internal component.Examples of passages disposed through a structure such as a liquiddistributor of a distillation column include drain holes, drain tubes,drain slits, and the like, which allow a liquid to flow through thestructure from one side to another.

Average residence time is defined as the sum total of all liquid volumehold-up for a given phase within a distillation zone divided by theaverage flow rate of that phase through the distillation zone. Thehold-up volume for a given phase can include liquid volume contained inthe various internal components of the column including collectors,distributors and the like, as well as liquid contained on trays, withindowncomers, and/or within structured or random packed bed sections.

Treatment of Hydrogen Iodide Containing Streams

Some conventional carbonylation reaction mixtures have been found tocontain hydrogen iodide, which dissociates in the presence of watercausing corrosion within the reaction zone and the separation zone.Hydrogen iodide also creates environmental concerns as well ascost-related concerns. The presence of hydrogen iodide has beenrecognized in some process streams, e.g., reaction vent streams orflasher vent streams. Although some conventional separation trains doutilize absorbent towers, e.g., to treat methyl iodide, the use of thesetowers to treat the hydrogen iodide in these streams, e.g., recovery orremoval thereof, however, has not been addressed.

It has now been discovered that the hydrogen iodide in many of thesestreams may be treated using an absorbent (or multiple absorbents). Theuse of the absorbent effectively removes hydrogen iodide from variousprocess streams, e.g., reactor vents, flasher vents, light ends columnvents, and PRC removal system vents. The absorption using appropriateabsorbents surprisingly and unexpectedly separates the hydrogen iodideand allows for proper treatment and/or recycling thereof. In particularcases, the inventors have found that the use of a single absorbent isadvantageous to treat hydrogen iodide. Preferably, the absorbentcomprises acetic acid. The use of a single absorbent allows theresultant extract stream to then be purified by directly conveying theextractant stream back to the separation zone. In some cases, the use ofa single absorbent eliminates the need to further treat the resultantextract before returning it back to the separation zone.

Also, conventional systems and literature have not addressed thepresence of hydrogen iodide in the overhead streams, bottoms streams,and/or accompanying receivers of a PRC removal system. The inventorshave now found that, in some cases, the hydrogen iodide in theseoverhead streams is significant. Without being bound by theory, it ispostulated that the PRC removal process actually produces hydrogeniodide via hydrolysis of methyl iodide. Thus, the amount of hydrogeniodide in the overhead streams and bottoms streams of a PRC removalsystem become enriched in hydrogen iodide. The higher levels of hydrogeniodide creates and exacerbates the environmental, corrosion, andcost-related concerns mentioned above. The inventors have found thattreatment of these streams using an absorbent (or multiple absorbents)surprisingly and effectively provides for effective removal of hydrogeniodide from these streams. For environmental as well as cost-relatedreasons.

In addition, it has now been discovered that the use of an absorbertower that employs the (alternating) use of multiple scrubber solventscan be used to effectively separate hydrogen iodide from any of severalacetic acid production process streams. Exemplary process streams thatmay be used as the absorber tower feed include a reactor vent stream, aflasher vent stream, a light ends distillate, a PRC removal systemoverhead (or a vent from an accompanying receiver) and derivatives ofthese process streams. The specific combination of scrubbing solvents,as described herein, effectively remove hydrogen iodide from therespective process stream advantageously decreasing the corrosiveeffects thereof. As a result, metallurgical problems throughout thereaction and separation zones are minimized. In addition, it has beensurprisingly found that the use of the specific solvents of the presentinvention may beneficially lead to the formation of additional methyliodide, which can then be utilized to increase catalyst stability in thereaction zone (or elsewhere). Without being bound by theory, whenmethanol and/or methyl acetate are used as a scrubbing solvent, e.g., asa second scrubbing solvent, the methanol and/or methyl acetate may reactwith hydrogen iodide in the various acetic acid production processstreams to form the additional methyl iodide. The processes of thepresent invention improve the purification of the crude acetic acidproduct by improving hydrogen iodide removal, increasing methyl iodideformation, and beneficially improving overall catalyst stability.

The present invention relates to processes for producing acetic acidcomprising the step of carbonylating, in a reactor, at least one ofmethanol, dimethyl ether, and methyl acetate in a reaction mediumcomprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and water, to form a crude acetic acid product.The crude acetic acid product comprises acetic acid. During thecarbonylation reaction, a vent stream may be vented from the reactor.The reactor vent stream may comprise hydrogen iodide, carbon monoxide,nitrogen, and other volatile components. The crude acetic acid productis flashed, with or without heat, to form a first vapor stream and afirst liquid residue stream. The first vapor stream comprises aceticacid, hydrogen iodide, and optionally methyl iodide. The first liquidresidue stream comprises metal catalyst and halide salt and may berecycled to the reactor. The flashed vapor stream (or a portion thereof)is separated in a light ends column to form a second vapor stream, aside draw, and a second liquid residue. The second vapor streamcomprises hydrogen iodide and optionally methyl iodide and acetaldehyde.The second vapor stream may further comprise volatile components, e.g.,vaporous acetic acid, water, and/or methyl acetate. The side drawcomprises purified acetic acid product, which may be subjected toadditional purification steps. The second residue stream comprisesacetic acid, water, and catalyst and may be recycled to the reactor. Theseparating in the light ends column may further comprise condensing atleast a portion of the second vapor stream to form a light ends ventstream comprising hydrogen iodide.

The processes further comprise the step of feeding to the absorber towerat least a portion of the reactor vent stream, at least a portion of theflasher vent stream, and/or at least a portion of the second vaporstream.

In one embodiment, at least a portion of the reactor vent stream may befed to the absorber column. A portion of the reactor vent stream may beseparated from the reactor vent stream and fed to the absorber. Theentire reactor vent stream may be fed to the absorber. In oneembodiment, the portion of the reactor vent stream that is fed to theabsorber is formed by directing the reactor vent stream to a heatexchanger, e.g., condensing at least a portion of the reactor ventstream, which yields a reactor vent vapor stream and a condensed reactorreturn stream. The reactor vent vapor stream may comprise hydrogeniodide and may be fed to the absorber tower. The condensed reactorreturn stream may be recycled to the reactor.

In one embodiment, at least a portion of the first vapor stream may befed to the absorber column. A portion of the first vapor stream may beseparated from the first vapor stream and fed to the absorber. Theentire first vapor stream may be fed to the absorber. In one embodiment,the portion of the first vapor stream that is fed to the absorber isformed by directing the first vapor stream to a heat exchanger, e.g.,condensing at least a portion of the first vapor stream, which yields aflasher vent vapor stream and a condensed flasher return stream. Theflasher vent vapor stream may comprise hydrogen iodide and may be fed tothe absorber tower. The condensed flasher return stream may be recycledto the flasher or the reactor.

In one embodiment, at least a portion of the second vapor stream may befed to the absorber column. A portion of the second vapor stream isseparated from the second vapor stream and fed to the absorber. Theentire second vapor stream is fed to the absorber. In one embodiment,the portion of the second vapor stream that is fed to the absorber isformed by directing the second vapor stream to a heat exchanger, e.g.,condensing at least a portion of the second vapor stream, which yields alight ends vent vapor stream and a condensed light ends return stream.The light ends vent vapor stream may comprise hydrogen iodide and may befed to the absorber tower. The condensed light ends return stream may berecycled to the light ends column, the flasher, or the reactor.

In one embodiment, at least a portion of the second vapor stream may bedecanted to form a decanter vent stream. The decanter vent stream maycomprise hydrogen iodide and may be fed to the absorber tower.

In one embodiment. at least a portion of at least two of the reactorvent stream, the flasher vent stream, the second vapor stream, and a PRCremoval system vent are combined to form the absorber tower feed. Forexample, the reactor vent stream, the flasher vent stream, and thesecond vapor stream may be combined into a combined vent stream and fedto the absorber. As another example, the flasher vent stream, the secondvapor stream, and the PRC removal system vent may be combined into acombined vent stream and fed to the absorber. Thus, the absorber towerfeed may, in some embodiments, comprise the combined vent stream.

Regardless of which stream is (streams are) sent to the absorber column,the processes further comprise the step of contacting the absorber towerfeed with a first absorbent. The first absorbent preferably absorbsmethyl iodide from the absorber tower feed to form a first extract,which comprises the first absorbent and the absorbed methyl iodide. Thefirst absorbent may also preferably absorb hydrogen iodide from theabsorber tower feed to form the first extract, which comprises the firstabsorbent and the absorbed hydrogen iodide. The first absorbent maycomprise acetic acid, water, or combinations thereof

The process further comprises the steps of conveying all or a portion ofthe first extract to the light ends column and/or drying column andterminating the supply of first absorbent to the absorber tower. Forexample, the flow of the first absorbent to the absorber column may bestopped using a system of valves, as discussed below.

As used herein, the term “terminating” includes a reducing or decreasingthe supply of an absorbent.

The processes, in one embodiment, further comprise the step ofcontacting the absorber tower feed with a second absorbent. The secondabsorbent may absorb methyl iodide and hydrogen iodide and thus forms asecond extract, which comprises the second absorbent and absorbed methyliodide. The second absorbent may comprise methanol and/or methylacetate. In one embodiment, the second absorbent consists essentially ofmethanol. The processes may further comprise the step of conveying allor a portion of the second extract to the reactor. The conveying of thesecond extract to the reactor may be performed after the termination ofthe supply of first absorbent. In one embodiment, the contacting of theabsorber tower feed with the methanol or methyl acetate in the secondabsorbent beneficially forms methyl iodide, which can be utilized toincrease catalyst stability in the reactor. In one embodiment, thesecond extract is mixed with methanol or a reactive derivative thereofprior to being conveyed to the reactor. The second extract is returnedto the reactor and is not returned to the separation train for furtherprocessing. By sending the absorber stream to the reactor, there is noneed for a separate stripper column or processing in light ends ordrying column which consumes considerable capacity. As such, the burdenon the separation zone is beneficially reduced.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; optionallyventing from the reactor a reactor vent stream comprising hydrogeniodide; condensing at least a portion of the reactor vent stream to forma condensed reactor vent stream; feeding to an absorber tower anabsorber tower feed comprising at least a portion of the condensedreactor vent stream; contacting the absorber tower feed with a firstabsorbent selected from the group consisting of acetic acid, methanol,and methyl acetate, preferably acetic acid, to absorb methyl iodide andhydrogen iodide and to form a first extract comprising the firstabsorbent and the absorbed methyl iodide and hydrogen iodide; conveyingthe first extract, directly or indirectly, to the light ends columnand/or drying column; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor. Thus, two absorbentsteps are employed in this embodiment.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, optionallycondensing a portion of the first vapor stream to form a condensedflasher return stream and a flasher vent stream comprising hydrogeniodide; feeding to an absorber tower an absorber tower feed comprisingat least a portion of the flasher vent stream (or the condensed flashervent stream); contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate, preferably acetic acid, to absorb methyl iodide and hydrogeniodide and to form a first extract comprising the first absorbent andthe absorbed methyl iodide and hydrogen iodide; conveying the firstextract, directly or indirectly, to the light ends column and/or dryingcolumn; decreasing the supply of first absorbent to the absorber tower;contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide; conveying the second extract, directly orindirectly, to the reactor. Two absorbent steps are employed in thisembodiment.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt; separating, ina light ends column, the flashed first vapor stream to form a secondvapor stream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; optionally condensing the second vapor stream, feeding to anabsorber tower an absorber tower feed comprising at least a portion ofthe second vapor stream; contacting the absorber tower feed with a firstabsorbent selected from the group consisting of acetic acid, methanol,and methyl acetate, preferably acetic acid, to absorb methyl iodide andhydrogen iodide and to form a first extract comprising the firstabsorbent and the absorbed methyl iodide and hydrogen iodide; conveyingthe first extract, directly or indirectly, to the light ends columnand/or drying column; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor. Two absorbent steps areemployed in this embodiment.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, separating, ina light ends column, the flashed first vapor stream to form a secondvapor stream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; decanting the second vapor stream to form a light phasecomprising acetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;separating at least a portion of the light phase and/or the heavy phaseto a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiver;optionally venting from the overhead receiver a PRS overhead receivervent stream comprising hydrogen iodide; feeding to an absorber tower anabsorber tower feed comprising at least a portion of the PRS overheadreceiver vent stream and/or the PRS overhead; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate, preferably acetic acid, toabsorb methyl iodide and hydrogen iodide to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide. Two absorbent steps areemployed in this embodiment.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, optionallycondensing a portion of the first vapor stream to form a condensedflasher return stream and a flasher vent stream comprising hydrogeniodide; separating, in a light ends column, the flashed first vaporstream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a first column and asecond column to obtain a second PRS overhead comprising hydrogen iodidefrom the second PRC removal column; optionally collecting the second PRSoverhead in an overhead receiver and venting from the overhead receivera second PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second PRS overhead receiver vent stream and/or thesecond PRS overhead; contacting the absorber tower feed with a firstabsorbent selected from the group consisting of acetic acid, methanol,and methyl acetate to absorb hydrogen iodide to form a first extractcomprising absorbed hydrogen iodide; and optionally contacting the

absorber tower feed with a second absorbent comprising methanol and/ormethyl acetate to absorb methyl iodide and hydrogen iodide and to form asecond extract comprising the second absorbent and the absorbed methyliodide.

Other embodiments that employ a single absorbent are also contemplated.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, optionallycondensing a portion of the first vapor stream to form a condensedflasher return stream and a flasher vent stream comprising hydrogeniodide; separating, in a light ends column, the flashed first vaporstream (or the condensed first vapor stream) to form a second vaporstream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; decanting the second vapor stream to form a light phasecomprising acetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;separating at least a portion of the light phase and/or the heavy phasein a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiverand optionally venting from the overhead receiver a PRS overheadreceiver vent stream comprising hydrogen iodide; feeding to an absorbertower an absorber tower feed comprising at least a portion of the PRSoverhead receiver vent stream and/or the PRS overhead; contacting theabsorber tower feed with a first absorbent selected from the groupconsisting of acetic acid, methanol, and methyl acetate, preferablyacetic acid, to absorb hydrogen iodide to form a first extractcomprising absorbed hydrogen iodide; and optionally conveying the secondextract, directly or indirectly, to the reactor.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, optionallycondensing a portion of the first vapor stream to form a condensedflasher return stream and a flasher vent stream comprising hydrogeniodide; separating, in a light ends column, the flashed first vaporstream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a single column toform a PRS overhead comprising hydrogen iodide; optionally collectingthe PRS overhead in an overhead receiver and venting from the overheadreceiver a PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead; contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate to absorb hydrogen iodide to form a first extract comprisingabsorbed hydrogen iodide; and optionally contacting the absorber towerfeed with a second absorbent comprising methanol and/or methyl acetateto absorb methyl iodide and hydrogen iodide and to form a second extractcomprising the second absorbent and the absorbed methyl iodide.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt, optionallycondensing a portion of the first vapor stream to form a condensedflasher return stream and a flasher vent stream comprising hydrogeniodide; feeding to an absorber tower an absorber tower feed comprisingat least a portion of the flasher vent stream (or the condensed flashervent stream); contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate, preferably acetic acid, to absorb methyl iodide and hydrogeniodide to form a first extract comprising the first absorbent, hydrogeniodide, and methyl iodide.

In one embodiment, the process comprises the steps of: carbonylating, ina reactor, at least one of methanol, dimethyl ether, and methyl acetatein a reaction medium comprising a metal catalyst, methyl iodide, aniodide salt, and optionally acetic acid and a finite amount water, toform a crude acetic acid product comprising acetic acid; flashing thecrude acetic acid product, with or without heat, to form a first vaporstream comprising acetic acid and methyl iodide and a first liquidresidue stream comprising metal catalyst and halide salt; separating, ina light ends column, the flashed first vapor stream (or the condensedflasher vent stream) to form a second vapor stream comprising methyliodide and hydrogen iodide, a side draw comprising purified acetic acidproduct, and a second liquid residue stream; feeding to an absorbertower an absorber tower feed comprising at least a portion of the secondvapor stream; contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate, preferably acetic acid, to absorb methyl iodide and hydrogeniodide to form a first extract comprising the first absorbent, hydrogeniodide, and methyl iodide.

In cases where acetic acid is utilized as the single absorbent, theinventors have found that the hydrogen iodide is absorbed particularlywell when acetic acid, methanol, and/or methyl acetate are employed.These absorbents have not been shown in the literature to be effectivesolvents for hydrogen iodide removal.

By treating the hydrogen iodide-containing streams with the absorbent(s)as discussed above, hydrogen iodide is advantageously separated from therespective stream. Once removed, the hydrogen can beneficially befurther utilized or disposed of accordingly.

The processes may further comprise the step of chilling the firstabsorbent and/or the second absorbent prior to contacting secondabsorbent with the absorber tower feed.

In one embodiment where multiple absorbents are employed, the absorbertower feed comprises methyl acetate and the first and second absorbentsabsorb methyl acetate from the absorber tower feed. The methyl acetatein the absorber tower feed may be provided by one or more of the reactorvent stream, the flasher vent stream, and the second vapor stream.

The inventors have further found that, advantageously, the use of twoscrubbing solvents, as described herein, eliminates the need fortreatment of the first absorbent and/or the second absorbent. Instead,these streams can be directed to existing components of the process,e.g., the reactor, the light ends column, or the drying column.

In cases where a reactor vent is taken from the reactor, the resultingstream may be directed to the flasher to stabilize catalyst. Preferably,the reactor vent is directed to the bottom of the flasher to stabilizethe catalyst present in the stream exiting the bottom of the flasher.

In one embodiment where multiple absorbents are employed, the absorbertower alternates between using the first absorbent and the secondabsorbent. In some cases, a transition period exists between thetermination of the use of the first absorbent and the beginning of theuse of the second absorbent and vice versa. In some cases, thetransition period may be less than 20 minutes, e.g., less than 15minutes, less than 10 minutes, less than 5 minutes, or less than 3minutes. In one embodiment, acetic acid is used as an absorbent during aunit startup because the acetic acid can be directed to the purificationsystem that has been operating before the reactor is started. Withoutbeing bound by theory, it is believed that if methanol were used beforethe reactor was running, there would be nowhere for the methanol to besubsequently processed. The start-up period using acetic acid lasts forseveral hours, e.g., less than 15 hours, less than 10 hours, or lessthan 5 hours. In one embodiment, the conveying of the second absorberreturn stream comprises the step of conveying the second extract to thereactor during a transition period; and following the transition period,continuing to feed the second absorber return stream to the reactor. Inthese cases, once the transition has been made to the second absorbent,e.g., methanol, the absorber stream continues to feed to the reactorwhere the methanol is consumed as reactant and the MeI is returned tothe reactor.

In some cases, the processes relate to a process for starting up thereaction system. For example, the feeding to the absorber tower mayinitiate a start-up period, and during the start-up period the absorbertower feed is contacted with a first absorbent and the first extract isconveyed to the light ends column and/or drying column, as discussedherein. A transition from start up to steady operation may be initiated(during a changeover period) by terminating the supply of firstabsorbent to the absorber tower. The second absorbent may be provided tothe absorber tower. During at least a portion of the changeover period,acetic acid, methanol, and methyl iodide may be provided to the absorbertower, thus forming a combined absorber return stream comprising aceticacid, methanol, and methyl iodide. The combined absorber return streammay be conveyed from the absorber tower to the reactor. Steady stateoperation may be initiated after the changeover period. During steadystate operation, the second vapor stream may be contacted with a secondabsorbent, as discussed herein. During the steady state operation, thesecond absorber return stream may be mixed with methanol or a reactivederivative thereof to form a mixed stream, and the mixed stream may beconveyed to the reactor. In one embodiment, termination of supply offirst absorbent to the absorber tower and providing of the secondabsorbent to the absorber tower occur substantially simultaneously,e.g., within less than 1 minute, within less than 5 minutes, or withinless than 20 minutes.

Acetic Acid Production Systems

An exemplary acetic acid production process is described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. The process description is merelyexemplary and is not meant to limit the scope of the invention.

The carbonylation reaction of methanol to acetic acid product may becarried out by contacting the methanol feed with gaseous carbon monoxidebubbled through an acetic acid solvent reaction medium containing themetal catalyst, e.g., rhodium catalyst, a halogen-containing catalystpromoter, e.g., methyl iodide, additional soluble halide salt, e.g.,iodide salt such as lithium iodide, and optionally methyl acetate and/orwater, at conditions of temperature and pressure suitable to form thecarbonylation product.

The catalyst may be a Group VIII metal catalyst, such as rhodium, and ahalogen-containing catalyst promoter. A particularly useful process isthe low water rhodium-catalyzed carbonylation of methanol to acetic acidas exemplified in U.S. Pat. No. 5,001,259. Other metal catalysts, e.g.,iridium-based catalysts, are contemplated as well. Generally, the metalcomponent, e.g., rhodium component, of the catalyst system is believedto be present in the form of a coordination compound of rhodium with ahalogen component providing at least one of the ligands of suchcoordination compound. In addition to the coordination of rhodium andhalogen, it is also believed that carbon monoxide will coordinate withrhodium. The rhodium component of the catalyst system may be provided byintroducing into the reaction zone rhodium in the form of rhodium metal,rhodium salts such as the oxides, acetates, iodides, carbonates,hydroxides, chlorides, etc., or other compounds that result in theformation of a coordination compound of rhodium in the reactionenvironment. The metal catalyst, e.g., rhodium catalyst, is, in someembodiments, present in amounts from 200 to 2000 weight parts permillion (wppm).

The halogen-containing catalyst promoter of the catalyst system consistsof a halogen compound comprising an organic halide. Thus, alkyl, aryl,and substituted alkyl or aryl halides can be used. Preferably, thehalogen-containing catalyst promoter is present in the form of an alkylhalide. Even more preferably, the halogen-containing catalyst promoteris present in the form of an alkyl halide in which the alkyl radicalcorresponds to the alkyl radical of the feed alcohol, which is beingcarbonylated. Thus, in the carbonylation of methanol to acetic acid, thehalide promoter will include methyl halide, and more preferably methyliodide.

It will be generally recognized that it is the concentration of iodideion in the catalyst system that is important and not the cationassociated with the iodide, and that at a given molar concentration ofiodide the nature of the cation is not as significant as the effect ofthe iodide concentration. Any metal iodide salt, or any iodide salt ofany organic cation, or other cations such as those based on amine orphosphine compounds (optionally, ternary or quaternary cations), can bemaintained in the reaction medium provided that the salt is sufficientlysoluble in the reaction medium to provide the desired level of theiodide. When the iodide is a metal salt, preferably it is an iodide saltof a member of the group consisting of the metals of Group IA and GroupIIA of the periodic table as set forth in the “Handbook of Chemistry andPhysics” published by CRC Press, Cleveland, Ohio, 2002-03 (83rdedition). In particular, alkali metal iodides are useful, with lithiumiodide being particularly suitable. In the low water carbonylationprocess, the additional iodide ion over and above the iodide ion presentas hydrogen iodide is generally present in the catalyst solution inamounts such that the total iodide ion concentration is from 2 to 20 wt.% and the methyl acetate is generally present in amounts from 0.5 to 30wt. %, and the methyl iodide is generally present in amounts from 5 to20 wt. %.

In some embodiments, the desired reaction rates are obtained even at lowwater concentrations by maintaining in the reaction medium an ester ofthe desired carboxylic acid and an alcohol, desirably the alcohol usedin the carbonylation, and an additional iodide ion that is over andabove the iodide ion that is present as hydrogen iodide. A desired esteris methyl acetate. It has been found, as described in U.S. Pat. No.5,001,259, that under low water concentrations, methyl acetate andlithium iodide act as rate promoters only when relatively highconcentrations of each of these components are present and that thepromotion is higher when both of these components are presentsimultaneously. The absolute concentration of iodide ion contentprovided as merely an example, and is not to be interpreted as limiting.

In embodiments, the process for producing acetic acid further includesintroducing a lithium compound into the reactor to maintain theconcentration of lithium acetate in an amount from 0.3 to 0.7 wt. % inthe reaction medium. In embodiments, an amount of the lithium compoundis introduced into the reactor to maintain the concentration of hydrogeniodide in an amount from 0.1 to 1.3 wt. % in the reaction medium. Inembodiments, the concentration of the rhodium catalyst is maintained inan amount from 200 to 3000 wppm in the reaction medium, theconcentration of water is maintained in amount from 0.1 to 4.1 wt. % inthe reaction medium, and the concentration of methyl acetate ismaintained from 0.6 to 4.1 wt. % in the reaction medium, based on thetotal weight of the reaction medium present within the carbonylationreactor.

In embodiments, the lithium compound introduced into the reactor isselected from the group consisting of lithium acetate, lithiumcarboxylates, lithium carbonates, lithium hydroxide, other organiclithium salts, and mixtures thereof. In embodiments, the lithiumcompound is soluble in the reaction medium. In an embodiment, lithiumacetate dihydrate may be used as the source of the lithium compound.

Lithium acetate reacts with hydrogen iodide according to the followingequilibrium reaction (I) to form lithium iodide and acetic acid:

LiOAc+HI

LiI+HOAc  (I)

Lithium acetate is thought to provide improved control of hydrogeniodide concentration relative to other acetates, such as methyl acetate,present in the reaction medium. Without being bound by theory, lithiumacetate is a conjugate base of acetic acid and thus reactive towardhydrogen iodide via an acid-base reaction. This property is thought toresult in an equilibrium of the reaction (I) which favors reactionproducts over and above that produced by the corresponding equilibriumof methyl acetate and hydrogen iodide. This improved equilibrium isfavored by water concentrations of less than 4.1 wt. % in the reactionmedium. In addition, the relatively low volatility of lithium acetatecompared to methyl acetate allows the lithium acetate to remain in thereaction medium except for volatility losses and small amounts ofentrainment into the vapor crude product. In contrast, the relativelyhigh volatility of methyl acetate allows the material to distill intothe purification train, rendering methyl acetate more difficult tocontrol. Lithium acetate is much easier to maintain and control in theprocess at consistent low concentrations of hydrogen iodide.Accordingly, a relatively small amount of lithium acetate may beemployed relative to the amount of methyl acetate needed to controlhydrogen iodide concentrations in the reaction medium. It has furtherbeen discovered that lithium acetate is at least three times moreeffective than methyl acetate in promoting methyl iodide oxidativeaddition to the rhodium [I] complex.

In embodiments, the concentration of lithium acetate in the reactionmedium is maintained at greater than or equal to 0.3 wt. %, or greaterthan or equal to 0.35 wt. %, or greater than or equal to 0.4 wt. %, orgreater than or equal to 0.45 wt. %, or greater than or equal to 0.5 wt.%, and/or in embodiments, the concentration of lithium acetate in thereaction medium is maintained at less than or equal to 0.7 wt. %, orless than or equal to 0.65 wt. %, or less than or equal to 0.6 wt. %, orless than or equal to 0.55 wt. %.

It has been discovered that an excess of lithium acetate in the reactionmedium can adversely affect the other compounds in the reaction medium,leading to decrease productivity. Conversely, it has been discoveredthat a lithium acetate concentration in the reaction medium below about0.3 wt. % is unable to maintain the desired hydrogen iodideconcentrations in the reaction medium of below 1.3 wt. %.

In embodiments, the lithium compound may be introduced continuously orintermittently into the reaction medium. In embodiments, the lithiumcompound is introduced during reactor start up. In embodiments, thelithium compound is introduced intermittently to replace entrainmentlosses.

The reaction medium may also contain impurities that should becontrolled to avoid byproduct formation. One impurity in the reactionmedium may be ethyl iodide, which is difficult to separate from aceticacid. Applicant has further discovered that the formation of ethyliodide may be affected by numerous variables, including theconcentration of acetaldehyde, ethyl acetate, methyl acetate and methyliodide in the reaction medium. Additionally, ethanol content in themethanol source, hydrogen partial pressure and hydrogen content in thecarbon monoxide source have been discovered to affect ethyl iodideconcentration in the reaction medium and, consequently, propionic acidconcentration in the final acetic acid product.

In embodiments, the propionic acid concentration in the acetic acidproduct may further be maintained below 250 wppm by maintaining theethyl iodide concentration in the reaction medium at less than or equalto 750 wppm without removing propionic acid from the acetic acidproduct.

In embodiments, the ethyl iodide concentration in the reaction mediumand propionic acid in the acetic acid product may be present in a weightratio from 3:1 to 1:2. In embodiments, the acetaldehyde:ethyl iodideconcentration in the reaction medium is maintained at a weight ratiofrom 2:1 to 20:1.

In embodiments, the ethyl iodide concentration in the reaction mediummay be maintained by controlling at least one of the hydrogen partialpressure, the methyl acetate concentration, the methyl iodideconcentration, and/or the acetaldehyde concentration in the reactionmedium.

In embodiments, the concentration of ethyl iodide in the reaction mediumis maintained/controlled to be less than or equal to 750 wppm, or e.g.,less than or equal to 650 wppm, or less than or equal to 550 wppm, orless than or equal to 450 wppm, or less than or equal to 350 wppm. Inembodiments, the concentration of ethyl iodide in the reaction medium ismaintained/controlled at greater than or equal to 1 wppm, or e.g., 5wppm, or 10 wppm, or 20 wppm, or 25 wppm, and less than or equal to 650wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.

In embodiments, the weight ratio of ethyl iodide in the reaction mediumto propionic acid in the acetic acid product may range from 3:1 to 1:2,or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from 3:2 to 1:2.

In embodiments, the weight ratio of acetaldehyde to ethyl iodide in thereaction medium may range from 20:1 to 2:1, or e.g., from 15:1 to 2:1 orfrom 9:1 to 2:1.

In one embodiment, the gaseous purge stream 106 contains low amounts ofhydrogen iodide of less than or equal to 1 wt. %, e.g., less than orequal to 0.9 wt. %, less than or equal to 0.8 wt. %, less than or equalto 0.7 wt. %, less than or equal to 0.5 wt. %. Hydrogen iodide in excessof these amounts may increase the duty on the scrubber to preventhydrogen iodide from being purged.

In one embodiment, a suitable potassium permanganate test is JIS K1351(2007).

The liquid reaction medium employed may include any solvent compatiblewith the catalyst system and may include pure alcohols, or mixtures ofthe alcohol feedstock and/or the desired carboxylic acid and/or estersof these two compounds. A preferred solvent and liquid reaction mediumfor the low water carbonylation process contains the desired carboxylicacid product. Thus, in the carbonylation of methanol to acetic acid, apreferred solvent system contains acetic acid.

Water is contained in the reaction medium but desirably at lowconcentrations for achieving sufficient reaction rates. It haspreviously been taught, e.g., in U.S. Pat. No. 3,769,329, that inrhodium-catalyzed carbonylation reactions, the addition of water exertsa beneficial effect upon the reaction rate. Thus, some commercialoperations are commonly run at water concentrations of greater than 14wt. %. However, in some embodiments, water concentrations of less thanor equal to 14 wt. %, e.g., less than or equal to 10 wt. %, less than orequal to 1 wt. % or less than or equal to 0.1 wt. %, may be utilized. Interms of ranges, the reaction medium may comprise water in an amountfrom 0.1 wt. % to 14 wt. %, e.g., from 0.2 wt. % to 10 wt. %, from 0.25wt. % to 5 wt. %, or from 0.25 wt. % to 4.1 wt. %, based on the totalweight of the reaction medium.

In one embodiment, the carbonylating is conducted while maintaining thereaction system at concentrations of 2 to 25% by weight iodide salt, 1to 20% by weight methyl iodide, 0.1 to 30% by weight methyl acetate, and0.1 to 10% by weight water.

Typical reaction temperatures for carbonylation will be from 150 to 250°C., with the temperature range of 180 to 225° C. being a preferredrange. The carbon monoxide partial pressure in the reactor can varywidely but is typically from 2 to 30 atmospheres, e.g., from 3 to 10atmospheres. Because of the partial pressure of by-products and thevapor pressure of the contained liquids, the total reactor pressure willrange from 15 to 40 atmospheres.

Exemplary reaction and acetic acid recovery system 100 is shown inFIG. 1. As shown, methanol-containing feed stream 101 and carbonmonoxide-containing feed stream 102 are directed to liquid phasecarbonylation reactor 104, in which the carbonylation reaction occurs.

Carbonylation reactor 104 is preferably either a stirred vessel orbubble-column type vessel, with or without an agitator, within which thereacting liquid or slurry contents are maintained, preferablyautomatically, a predetermined level, which preferably remainssubstantially constant during normal operation. Into carbonylationreactor 104, fresh methanol, carbon monoxide, and sufficient water arecontinuously introduced as needed to maintain suitable concentrations inthe reaction medium.

In a typical carbonylation process, carbon monoxide is continuouslyintroduced into the carbonylation reactor, desirably through adistribution plate below the agitator, which may be used to stir thecontents. The gaseous feed preferably is thoroughly dispersed throughthe reacting liquid by this stirring means. Gaseous purge stream 106desirably is vented from the reactor 104 to prevent buildup of gaseousby-products and to maintain a set carbon monoxide partial pressure at agiven total reactor pressure. The temperature of the reactor may becontrolled and the carbon monoxide feed is introduced at a ratesufficient to maintain the desired total reactor pressure. Stream 113comprising the liquid reaction medium exits reactor 104. At least aportion of gaseous purge stream 106 is directed to heat exchanger 103where at least a portion of gaseous purge stream 106 is condensed toform condensed reactor return stream 105 and reactor vent vapor stream107. At least a portion of gaseous purge stream 106, e.g., reactor ventvapor stream 107, is directed to absorber tower 109. Condensed reactorreturn stream 105 is recycled to reactor 104.

The crude acetic acid product may be purified in separation zone 108 torecover the acetic acid and recycle catalyst solution, methyl iodide,methyl acetate, and other system components within the process. Thus, arecycled catalyst solution, such as stream 110 from flasher 112, andoptionally one or more of recycle streams 114, 116, 118, and 120, alsoare introduced into the reactor 104. Of course, one or more of therecycle streams may be combined prior to being introduced into thereactor. The separation system also preferably controls water and aceticacid content in the carbonylation reactor, as well as throughout thesystem, and facilitates PRC removal.

The reaction medium is drawn off from the carbonylation reactor 104 at arate sufficient to maintain a constant level therein and is provided toflasher 112 via stream 113. In flasher 112, the crude product isseparated in a flash separation step to obtain a vapor product stream122 comprising acetic acid and less volatile stream 110 comprising acatalyst-containing solution (predominantly acetic acid containing therhodium and the iodide salt along with lesser quantities of methylacetate, methyl iodide, and water), which preferably is recycled to thereactor, as discussed above.

The vapor product stream 122 also comprises methyl iodide, methylacetate, hydrogen iodide, water, PRC's. Dissolved gases exiting thereactor and entering the flasher comprise a portion of the carbonmonoxide and may also contain gaseous by-products such as methane,hydrogen, and carbon dioxide. Such dissolved gases exit the flasher aspart of the overhead stream. At least a portion of vapor product stream122 is directed to heat exchanger 121, via line 122′, where the contentsof line 122′ are condensed to form condensed flasher return stream 123and flasher vent vapor stream 125. At least a portion of vapor productstream 122, e.g., flasher vent vapor stream 125, is directed to absorbertower 109. Condensed flasher return stream 123 is recycled to reactor104.

In one embodiment, the vapor product stream comprises acetic acid,methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen iodide.In one embodiment, the vapor product stream comprises acetic acid in anamount from 45 to 75 wt. %, methyl iodide in an amount from 20 to 50 wt.%, methyl acetate in an amount of less than or equal to 9 wt. %, andwater in an amount of less than or equal to 15 wt. %, based on the totalweight of the vapor product stream. In another embodiment, the vaporproduct stream comprises acetic acid in an amount from 45 to 75 wt. %,methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetatein an amount of less than or equal to 9 wt. %, and water in an amount ofless than or equal to 15 wt. %, based on the total weight of the vaporproduct stream. More preferably, the vapor product stream comprisesacetic acid in an amount from 55 to 75 wt. %, methyl iodide in an amountfrom 24 to 35 wt. %, methyl acetate in an amount from 0.5 to 8 wt. %,and water in an amount from 0.5 to 14 wt. %. In yet a further preferredembodiment, the vapor product stream comprises acetic acid in an amountfrom 60 to 70 wt. %, methyl iodide in an amount from 25 to 35 wt. %,methyl acetate in an amount from 0.5 to 6.5 wt. %, and water in anamount from 1 to 8 wt. %. The acetaldehyde concentration in the vaporproduct stream may be in an amount from 0.005 to 1 wt. %, based on thetotal weight of the vapor product stream, e.g., from 0.01 to 0.8 wt. %,or from 0.01 to 0.7 wt. %. In some embodiments the acetaldehyde may bepresent in amounts less than or equal to 0.01 wt. %. The vapor productstream may comprise hydrogen iodide in an amount less than or equal to 1wt. %, based on the total weight of the vapor product stream, e.g., lessthan or equal to 0.5 wt. %, or less than or equal to 0.1 wt. %. Hydrogeniodide is preferably present in the vapor product stream. The vaporproduct stream is preferably substantially free of, i.e., contains lessthan or equal to 0.0001 wt. %, propionic acid, based on the total weightof the vapor product stream.

The liquid recycle stream comprises acetic acid, the metal catalyst,corrosion metals, as well as other various compounds. In one embodiment,liquid recycle stream comprises acetic acid in an amount from 60 to 90wt. %, metal catalyst in an amount from 0.01 to 0.5 wt. %; corrosionmetals (e.g., nickel, iron and chromium) in a total amount from 10 to2500 wppm; lithium iodide in an amount from 5 to 20 wt. %; methyl iodidein an amount from 0.5 to 5 wt. %; methyl acetate in an amount from 0.1to 5 wt. %; water in an amount from 0.1 to 8 wt. %; acetaldehyde in anamount of less than or equal to 1 wt. % (e.g., from 0.0001 to 1 wt. %acetaldehyde); and hydrogen iodide in an amount of less than or equal to0.5 wt. % (e.g., from 0.0001 to 0.5 wt. % hydrogen iodide).

The overhead stream from flasher 112 is directed to the light endscolumn 124 as vapor product stream 122, where distillation yields alow-boiling overhead vapor stream 126, a purified acetic acid productthat preferably is removed via a side stream 128, and a high boilingresidue stream 116. Acetic acid removed via side stream 128 preferablyis subjected to further purification, such as in drying column 130 forselective separation of acetic acid from water. At least a portion ofoverhead vapor stream 126 is directed to heat exchanger 127, via line126, where the contents of line 126 are condensed to form condensedlight ends return stream 129 and light ends vent vapor stream 137. Atleast a portion of overhead vapor stream 126, e.g., light ends ventvapor stream 137, is directed to absorber tower 109. Condensed lightends return stream 129 is recycled to reactor 104 or optionally back tolight ends column 124 (not shown).

An exemplary absorber tower and the accompanying components are shown inFIG. 2.

One or more of streams 107, 125, 136, and 137 are directed to absorbersystem 200. Absorber system 200 includes switching system 202 which hasa plurality of valves and pumps in order to selectively couple absorbersystem 200 to scrubber solvent sources and return the used extracts tothe desired point in the carbonylation system as hereinafter described.Note also reactor 104 may be directly vented to absorber system 200 ifnecessary.

Absorber system 200 comprises absorber tower 209 which is fed with ventgas via line 204 and with scrubber solvent via line 206. Line 204represents one or more vent streams, as shown in FIG. 1. Preferably thescrubber solvent is chilled with a chiller 208 prior to being fed toabsorber tower 209 wherein the solvent flows countercurrently withrespect to the vent gas, absorbing methyl iodide, hydrogen iodide, andadditional relative components before exiting the tower via return line210 and being returned to the carbonylation unit. The scrubbed vent gasexits the tower via line 212 and may be further processed.Alternatively, a second stage water scrub could be provided in absorbertower 209 if so desired. Preferably, more than 90% of the methyl iodideis removed from the vent gas. The scrubber fluid is generally chilled toa temperature from 5° C. to 25° C. prior to use in the tower, with theproviso that when acetic acid is used as the scrubber solvent, thetemperature of the solvent is preferably held at 17° C., or more toprevent freezing.

Switching system 202 includes a plurality of valves such as valves 214,216, 218, 220, 222 and one or more pumps 224, 226 to raise pressure inthe return lines 228, 230, 232, 234 if needed. Feed valves 220, 222 areused to select the scrubber solvent which may be methanol from a feedtank or product acetic acid depending upon the mode of operation ofabsorber tower 209.

In steady state operation of the carbonylation system of FIG. 1 valve222 is closed and methanol is fed from the feed tank through open valve220 via line 236 to chiller 208, wherein the methanol is cooled. Fromthe chiller, methanol is fed to absorber tower 209, where it flowscountercurrently with vent gas and sorbs methyl iodide and othervolatile components therefrom before exiting the column via line 210.The used solvent with sorbed methyl iodide is pumped back to the reactoror the feed tank with pumps 224, 226 via line 230. In this mode ofoperation valves 216, 218 are closed and valve 214 is open.

During start up or shut down of the system it may be desirable tooperate absorber tower 209 using acetic acid as the scrub solvent. Inthis mode of operation, valve 222 is open and valve 220 is closed. Acidmay be sourced from product acid or a tank if so desired. The acid flowsthrough line 236 to chiller 208 where it is chilled and fed to absorbertower 209 via line 206 and scrubs the vent gas supplied via line 204 asnoted above. The acid exits the absorber tower 209 via line 210 and ispumped back to the carbonylation system by way of pumps 224, 226 vialines 228, 232. In this mode of operation of absorber tower 209, valves214, 218 are closed and valve 216 is open so that the used acetic acidis returned to light ends column, the drying column, or elsewhere in thepurification system for stripping.

During changeover from one solvent to the other, such as from methanolto acetic acid, it is generally undesirable to return the scrub fluid tothe methanol feed system or light ends column since inefficienciesresult. For such, a changeover may be accomplished in from about 5 toabout 20 minutes, during which time the used scrubber solvent is fed toa catalyst reservoir. In changeover mode, valves 214, 216 are closed andvalve 218 is open. Thus the system is operated generally by way of (a)feeding vent gas from the carbonylation unit to the absorber tower, thevent gas including methyl iodide and optionally additional volatilecomponents; (b) supplying a first absorbent to the absorber tower, thefirst absorbent consisting essentially of acetic acid; (c) contactingthe vent gas with the first absorbent thereby removing methyl iodide andoptionally additional volatile components from the gas and absorbingmethyl iodide and optionally additional volatile components into thefirst absorbent; (d) feeding an absorber return stream including firstabsorbent and absorbed methyl iodide and optionally additional absorbedvolatile components to the light ends column, the drying column orelsewhere in the purification system; (e) terminating the supply offirst absorbent to the absorber tower; (f) supplying a second absorbentto the absorber tower, the second absorbent consisting essentially ofmethanol; (g) contacting the vent gas with the second absorbent therebyremoving methyl iodide and optionally additional volatile componentsfrom the gas and absorbing methyl iodide and optionally additionalvolatile components into the second absorbent; (h) feeding an absorberreturn stream including first absorbent, second absorbent, absorbedmethyl iodide and optionally additional absorbed volatile componentsfrom the absorber tower to the reactor; and (i) following the transitionperiod, continue feeding an absorber return stream including secondabsorbent and absorbed methyl iodide and optionally additional absorbedvolatile components to the reactor. Feed to the absorber tower isselected by operation of valves 220, 222.

Returning to the decanter operations, it has been disclosed in U.S. Pat.Nos. 6,143,930 and 6,339,171 that there is generally a higherconcentration of the PRC's, and in particular acetaldehyde content, inthe low-boiling overhead vapor stream exiting the light ends column thanin the high-boiling residue stream exiting the column. Thus, in somecases, low-boiling overhead vapor stream 126, containing PRC's, issubjected to additional processing in PRC removal system 132 to reduceand/or remove the amount of PRC's present. As shown, low-boilingoverhead vapor stream 126, therefore, is condensed and directed to anoverhead phase separation unit, as shown by overhead receiver decanter134. In addition to PRC's, low-boiling overhead vapor stream 126 maytypically contain methyl iodide, methyl acetate, acetic acid, and water.

Conditions are desirably maintained in the process such that low-boilingoverhead vapor stream 126, once in decanter 134, may separate to form alight phase and a heavy phase. Generally, low-boiling overhead vaporstream 126 is cooled to a temperature sufficient to condense andseparate the condensable methyl iodide, methyl acetate, acetaldehyde andother carbonyl components, and water to form two separate phases. Aportion of stream 126 may include noncondensable gases such as carbonmonoxide, carbon dioxide, hydrogen iodide, hydrogen, and the like thatcan be directed to absorber tower 109.

The condensed light phase in the decanter generally may comprise water,acetic acid, and PRC's, as well as quantities of methyl iodide andmethyl acetate. Hydrogen iodide may also be present in the light phase.The condensed heavy phase in decanter 134 may generally comprise methyliodide, methyl acetate, and PRC's. Hydrogen iodide may also be presentin the heavy phase. The condensed heavy liquid phase in the decanter 134can be conveniently recirculated, either directly or indirectly, to thereactor 104 via stream 118. For example, a portion of this condensedheavy liquid phase can be recirculated to the reactor, with a slipstream (not shown), generally a small amount, e.g., from 5 to 40 vol. %,or from 5 to 20 vol. %, of the heavy liquid phase being directed to aPRS. This slip stream of the heavy liquid phase may be treatedindividually or may be combined with the condensed light liquid phasestream 138 for further distillation and extraction of carbonylimpurities.

Although the specific compositions of the light phase stream may varywidely, some preferred compositions are provided below in Table 1. Inone embodiment, hydrogen iodide is present in the light ends overhead,preferably in an amount ranging from 0.01 wt. % to 1 wt. %, e.g., from0.02 wt. % to 1 wt. %, or from 0.04 wt. % to 0.07 wt. %.

TABLE 1 Exemplary Light Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water 40-80  50-75  70-75  Methyl Acetate1-50 1-25 1-15 Acetic Acid 1-40 1-25 5-15 PRC's <5 <3 <1 Methyl Iodide<10 <5 <3 HI <1 <0.5 <0.1

In one embodiment, the overhead decanter is arranged and constructed tomaintain a low interface level to prevent an excess hold up of methyliodide. Although the specific compositions of heavy liquid phase mayvary widely, some exemplary compositions are provided below in Table 2.In one embodiment, hydrogen iodide is present in the heavy phase inaddition to the components listed below.

TABLE 2 Exemplary Heavy Liquid Phase from Light Ends Overhead conc. (Wt.%) conc. (Wt. %) conc. (Wt. %) Water 0.01-2  0.05-1  0.1-0.9 MethylAcetate 0.1-25 0.5-20 0.7-15  Acetic Acid 0.1-10 0.2-8  0.5-6   PRC's <5<3 <1 Methyl Iodide  40-98  50-95 60-85

The density of the heavy liquid phase 134 may be from 1.3 to 2, e.g.,from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described inU.S. Pat. No. 6,677,480, the measured density in the heavy liquid phase134 correlates with the methyl acetate concentration in the reactionmedium. As density decreases, the methyl acetate concentration in thereaction medium increases. In one embodiment of the present invention,heavy liquid phase 134 is recycled to the reactor and the light liquidphase 133 is controlled to be recycled through the same pump. It may bedesirable to recycle a portion of the light liquid phase 133 that doesnot disrupt the pump and maintains a density of the combined lightliquid phase 133 and heavy liquid phase of greater than or equal to 1.3,e.g., greater than or equal to 1.4, greater than or equal to 1.5, orgreater than or equal to 1.7. As described herein, a portion of theheavy liquid phase 134 may be treated to remove impurities such asacetaldehyde.

As shown in FIG. 1, the light phase exits decanter 134 via stream 131. Afirst portion, e.g., aliquot portion, of light phase stream 131 isrecycled to the top of the light ends column 124 as reflux stream 140. Asecond portion, e.g., aliquot portion, of light phase stream 131 isdirected to PRS 132, as discussed below and as shown by stream 142. Athird portion, e.g., aliquot portion, of the light phase stream 131optionally may be recycled to reactor 104 as shown by recycle stream114, when additional water is desired or needed in reactor 104. Inpreferred aspects the water level is maintained in the reactor at adesired level without recycling stream 114 to reactor 104 sincerecycling stream 114 to the reactor undesirably may result in therecycle of acetic acid and unnecessarily increasing the load on reactor104. Thus, a recycle from decanter 134 to reactor 104 is through theheavy phase stream 118.

Light ends column 124 also preferably forms residuum or bottoms stream116, which comprises primarily acetic acid and water. Since light endsbottoms stream 116 typically may comprise some residual catalyst, it maybe beneficial to recycle all or a portion of light ends bottoms stream116 to reactor 104. Optionally, light ends bottoms stream 116 may becombined with the catalyst phase 110 from flasher 112 and returnedtogether to reactor 104, as shown in FIG. 1.

As indicated above, in addition to the overhead phase, the light endscolumn 124 also forms an acetic acid side stream 128, which preferablycomprises primarily acetic acid and water. In order to maintain anefficient product separation, it is important that the composition ofthe side stream 128 does not vary or fluctuate significantly duringnormal operation.

Optionally, a portion of the side stream 128 may be recirculated to thelight ends column, preferably to a point below where side stream 128 wasremoved from light ends column, in order to improve the separation.

Since side stream 128 contains water in addition to acetic acid, sidestream 128 from the light ends column 124 preferably is directed todrying column 130, in which the acetic acid and water are separated fromone another. As shown, drying column 130, separates acetic acid sidestream 128 to form overhead stream 144 comprised primarily of water andbottoms stream 146 comprised primarily of acetic acid. Overhead stream144 preferably is cooled and condensed in a phase separation unit, e.g.,decanter 148, to form a light phase and a heavy phase. As shown, aportion of the light phase is refluxed, as shown by streams 150 and 152and the remainder of the light phase is returned to the reactor 104, asshown by stream 120. The heavy phase, which typically is an emulsioncomprising water and methyl iodide, preferably is returned in itsentirety to the reactor 104, as shown by stream 122, optionally afterbeing combined with stream 120. Exemplary compositions for the lightphase of the drying column overhead are provided below in Table 3. Inone embodiment, hydrogen iodide is present in the drying column overheadin an amount ranging from 0.01 wt. % to 1 wt. %, e.g., from 0.05 wt. %to 1 wt. %, or from 0.01 wt. % to 0.5 wt. %.

TABLE 3 Exemplary Light Compositions from Drying Column Overhead conc.(Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-20 1-15 1-10 Water 50-90 60-90  70-90  MeI <10 <5  <3  MeAc 1-20 1-15 1-10 HI  <1 <0.5 <0.2

Drying column bottoms stream 146 preferably comprises or consistsessentially of acetic acid. In preferred embodiments, drying columnbottoms stream 146 comprises acetic acid in an amount greater than 90wt. %, e.g., greater than 95 wt. % or greater than 98 wt. %. Optionally,drying column bottoms stream 146 may be further processed, e.g., bypassing through an ion exchange resin, prior to being stored ortransported for commercial use.

PRC Removal

In some cases, it may be advantageous to remove PRCs, primarilyaldehydes such as acetaldehyde, from a low-boiling overhead vapor streamof a light ends distillation column, more preferably from the condensedlight phase of a low-boiling overhead vapor stream 126 from light endsdistillation column 124. One or more of the streams from PRC removalsystem 132 may be returned to the system, e.g., recycled, eitherdirectly or indirectly. In some cases, no return streams from the PRCremoval system 132 are directed to the reactor 104 or to recycle linesto reactor 104. The PRC removal system preferably includes at least onedistillation column and at least one extraction column to reduce and/orremove PRCs. US Patent Publication No. 2011/0288333, which is herebyincorporated by reference, describes various PRC removal systemembodiments that may be employed with the present process.

The PRC removal system shown in FIG. 1 may contain a single extractionstep or may include multiple extraction stages, as described for examplein U.S. Pat. No. 7,223,886 and optionally including multistagecountercurrent extraction. According to various embodiments, one or morestreams derived from either or both (i) the PRC removal systemdistillation column and/or (ii) the PRC removal system extraction stage(collectively shown as stream 154), for example, may be returned to thesystem, e.g., either or both (i) the light ends removal column and/or(ii) the drying column of the separation system for the acetic acidproduction system. For example, a first portion, e.g., an aliquotportion, of a bottoms stream from a PRC removal system column may bedirected to light ends column 124 for further processing, or a secondportion, e.g., an aliquot portion, of a bottoms stream from a PRCremoval system column may be directed to drying column 130, preferablythe upper portion of drying column 130, for further processing. Asanother example, a raffinate from a PRS extraction unit, notablycontaining methyl iodide, may be returned to the system, e.g., lightends column or drying column or the raffinate may be added directly todecanter 134 and/or may be returned to reactor 104.

PRC Removal System

In some embodiments, a portion of light liquid phase and/or heavy liquidphase may be separated and directed to PRS to recover methyl iodide andmethyl acetate during the acetaldehyde removal. As shown in Tables 1 and2 above, light liquid phase and/or heavy liquid phase each contain PRC'sand the process may include removing carbonyl impurities, such asacetaldehyde, that deteriorate the quality of the acetic acid productand may be removed in suitable impurity removal columns and absorbers asdescribed in U.S. Pat. Nos. 6,143,930; 6,339,171; 7,223,883; 7,223,886;7,855,306; 7,884,237; 8,889,904; and US Pub. Nos. 2006/0011462, whichare incorporated herein by reference in their entirety. Carbonylimpurities, such as acetaldehyde, may react with iodide catalystpromoters to form alkyl iodides, e.g., ethyl iodide, propyl iodide,butyl iodide, pentyl iodide, hexyl iodide, etc. Also, because manyimpurities originate with acetaldehyde, it is desirable to removecarbonyl impurities from the liquid light phase.

The portion of light liquid phase and/or heavy liquid phase fed to theacetaldehyde or PRC removal system may vary from 1% to 99% of the massflow of either the light liquid phase 133 and/or heavy liquid phase 134,e.g., from 1 to 50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to 20%.Also in some embodiments, a portion of both the light liquid phase 133and heavy liquid phase 134 may be fed to the acetaldehyde or PRC removalsystem. The portion of the light liquid phase 133 not fed to theacetaldehyde or PRC removal system may be refluxed to the first columnor recycled to the reactor, as described herein. The portion of theheavy liquid phase 134 not fed to the acetaldehyde or PRC removal systemmay be recycled to the reactor. Although a portion of heavy liquid phase134 may be refluxed to the first column, it is more desirable to returnthe methyl iodide enriched heavy liquid phase 134 to the reactor.

In one embodiment at least a portion the light phase and/or the heavyphase is conveyed to the PRC removal system for separation to form anoverhead comprising hydrogen iodide. In one embodiment, the PRC removalsystem is a one column system. In such cases, the inventors have nowfound that the overhead exiting the one column may contain hydrogeniodide, e.g., an enhanced amount of hydrogen iodide due to the reactionthat occurs in PRC removal system via hydrolysis of methyl iodide(and/or via the reaction of methyl iodide and acetic acid to yieldhydrogen iodide and methyl acetate). This stream may be sent to theabsorber tower for treatment of the hydrogen iodide.

As shown in FIG. 3, at least a portion of the light phase in line 142and/or the heavy phase in line 118′ may be directed to PRC removalsystem 132, which comprises column 160. Column 10 forms vapor overhead162 and bottom process stream 164 comprising water, methyl acetate,and/or methanol. Vapor overhead 162 comprises hydrogen iodide, e.g.,hydrogen iodide formed via reactions that take place in column 160.Vapor overhead 162 may be passed through a heat exchanger/condenser (notshown) and collected in receiver 166. Receiver vent 168 comprises someof the hydrogen iodide in vapor overhead 162 and is drawn from receiver166 and conveyed to absorber 109. A portion of vapor overhead 162 may becombined with receiver vent 18 and directed to absorber 109 via line162′. Optionally, line 162′ may be conveyed directly to absorber 109without being combined with line 168. A portion of the condensed vaporoverhead stream may be returned to column 160 via and another portionmay be directed to further processing.

In other embodiments, the PRS system is a two column system. Theinventors have now found that the overhead exiting either column maycontain hydrogen iodide, e.g., an enhanced amount of hydrogen iodide.

In one embodiment, a portion of light liquid phase 133 and/or heavyliquid phase 134 is fed to a distillation column which enriches theoverhead thereof to have acetaldehyde and methyl iodide. In addition,the overhead may also be enriched in hydrogen iodide. Depending on theconfiguration, there may be two separate distillation columns, and theoverhead of the second column may be enriched in acetaldehyde and methyliodide. The overhead of the second may also be enriched in hydrogeniodide. Dimethyl ether, which may be formed in-situ, may also be presentin the overhead. The overhead may be subject to one or more extractionstages to remove a raffinate enriched in methyl iodide and anextractant. In some cases, the overhead(s) of the PRC removal system maybe treated remove carbon monoxide. As one example, a pressure swingabsorber may be employed. A portion of the raffinate may be returned tothe distillation column, first column, overhead decanter and/or reactor.For example, when the heavy liquid phase is treated in the PRC removalsystem, it may be desirable to return a portion the raffinate to eitherthe distillation column or reactor. Also, for example, when light liquidphase 133 is treated in the PRC removal system, it may be desirable toreturn a portion the raffinate to either the first column, overheaddecanter, or reactor. In some embodiments, the extractant may be furtherdistilled to remove water, which is returned to the one or moreextraction stages. The column bottoms, which contains more methylacetate and methyl iodide than light liquid phase 133, may also berecycled to reactor 105 and/or refluxed to first column 120.

The overhead of a one column PRC removal unit may contain hydrogeniodide. The overhead may be collected in a receiver and a vent may bedrawn from the receiver.

In one embodiment, the process of the present invention comprises thesteps of: feeding to the absorber tower an absorber tower feedcomprising at least a portion of the PRS overhead receiver vent streamand/or the PRS overhead (from the first and only column); and contactingthe absorber tower feed with a first absorbent selected from the groupconsisting of acetic acid, methanol, and methyl acetate to absorbhydrogen iodide to form a first extract comprising absorbed hydrogeniodide.

In an embodiment wherein a two column system is employed, the process ofthe present invention comprises the steps of: feeding to an absorbertower an absorber tower feed comprising at least a portion of the PRSoverhead receiver vent stream and/or the PRS overhead (from the secondcolumn); contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate to absorb hydrogen iodide to form a first extract comprisingabsorbed hydrogen iodide.

In the embodiments where the overhead or receiver vent from the PRCremoval system is contacted with an absorbent, the inventors have foundthat the treatment with absorbent efficiently removes the hydrogeniodide that may be formed in the PRC removal system.

For purposes of the present specification and claims, the overheadstreams and overhead decanters of the light ends removal column and thedrying column are considered to be part of the light ends removal columnand of the drying column.

As indicated above, either phase of the low-boiling overhead vaporstream 126 may be subsequently processed to remove PRCs.

The distillation columns of the present invention may be a conventionaldistillation column, e.g., a plate column, a packed column, and others.Plate columns may include a perforated plate column, bubble-cap column,Kittel tray column, uniflux tray, or a ripple tray column. For a platecolumn, the theoretical number of plates is not particularly limited anddepending on the species of the component to be separate, may include upto 80 plates, e.g., from 2 to 80, from 5 to 60, from 5 to 50, or morepreferably from 7 to 35. The distillation column may include acombination of different distillation apparatuses. For example, acombination of bubble-cap column and perforated plate column may be usedas well as a combination of perforated plate column and a packed column.

The distillation temperature and pressure in the distillation system cansuitably be selected depending on the condition such as the species ofthe objective carboxylic acid and the species of the distillationcolumn, or the removal target selected from the lower boiling pointimpurity and the higher boiling point impurity according to thecomposition of the feed stream. For example, in a case where thepurification of acetic acid is carried out by the distillation column,the inner pressure of the distillation column (usually, the pressure ofthe column top) may be from 0.01 to 1 MPa, e.g., from 0.02 to 0.7 MPa,and more preferably from 0.05 to 0.5 MPa in terms of gauge pressure.Moreover, the distillation temperature for the distillation column,namely the inner temperature of the column at the temperature of thecolumn top, can be controlled by adjusting the inner pressure of thecolumn, and, for example, may be from 20 to 200° C., e.g., from 50 to180° C., and more preferably from 100 to 160° C.

The material of each member or unit associated with the distillationsystem, including the columns, valves, condensers, receivers, pumps,reboilers, and internals, and various lines, each communicating to thedistillation system may be suitable material such as glass, metal,ceramic, or combinations thereof, and is not particularly limited to aspecific one. According to the present invention, the material of theforegoing distillation system and various lines are a transition metalor a transition-metal-based alloy such as iron alloy, e.g., a stainlesssteel, nickel or nickel alloy, zirconium or zirconium alloy thereof,titanium or titanium alloy thereof, or aluminum alloy. Suitableiron-based alloys include those containing iron as a main component,e.g., a stainless steel that also comprises chromium, nickel, molybdenumand others. Suitable nickel-based alloys include those containing nickelas a main component and one or more of chromium, iron, cobalt,molybdenum, tungsten, manganese, and others, e.g., HASTELLOY™ andINCONEL™. Corrosion-resistant metals may be particularly suitable asmaterials for the distillation system and various lines.

For purposes of the present specification, it should be understood thatthe term “aliquot portion” refers to both: (i) a portion of a parentstream that has the same composition as the parent stream from which itis derived, and (ii) a stream comprising a portion of a parent streamthat has the same composition as the parent stream from which it isderived and one or more additional streams that have been combinedtherewith. Thus, directing a return stream comprising an aliquot portionof a PRS distillation bottoms stream to the light ends columnencompasses the direct transfer of a portion of the PRS distillationbottoms stream to the light ends column as well as the transfer of aderivative stream comprising (i) a portion of the PRS distillationbottoms stream and (ii) one or more additional streams that are combinedtherewith prior to introduction into the light ends column. An “aliquotportion” would not include, for example, streams formed in adistillation step or a phase separation step, which would not becompositionally the same as the parent stream from which they arederived nor derived from such a stream.

One of ordinary skill in the art having the benefit of this disclosurecan design and operate a PRS distillation column to achieve the desiredresults. Accordingly, the practice of this process is not necessarilylimited to specific characteristic of a particular distillation columnor the operation characteristics thereof, such as the total number ofstages, the feed point, reflux ratio, feed temperature, refluxtemperature, column temperature profile, and the like.

As is evident from the figures and text presented above, a variety ofembodiments are contemplated:

E1. A process for producing acetic acid comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid, optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of at least one of the reactor vent stream, the flasher ventstream, and the second vapor stream;

contacting the absorber tower feed with a first absorbent comprisingacetic acid to absorb methyl iodide and to form a first extractcomprising the first absorbent and the absorbed methyl iodide;

conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column;

terminating the supply of first absorbent to the absorber tower;

contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide;

conveying the second extract, directly or indirectly, to the reactor.

E2. The process according to embodiment E1, wherein the second absorbentconsists essentially of methanol.E3. The process according to embodiment E1 or E2, further comprising thestep of chilling the first absorbent prior to the contacting.E4. The process according to any one of embodiments E1 to E3, furthercomprising the step of chilling the second absorbent prior to thecontacting.E5. The process according to any one of embodiments E1 to E4, furthercomprising mixing the second absorber return stream with methanol or areactive derivative thereof prior to the conveying.E6. The process according to any one of embodiments E1 to E5, whereinthe absorber tower feed comprises methyl acetate and the first andsecond absorbents absorb methyl acetate from the absorber tower feed.E7. The process according to any one of embodiments E1 to E6, whereinthe process does not employ a stripper column to treat the firstabsorbent and/or the second absorbent.E8. The process according to any one of embodiments E1 to E7, whereinthe carbonylating is conducted while maintaining the reaction system atconcentrations of 2 to 25% by weight iodide salt, 1 to 20% by weightmethyl iodide, 0.1 to 30% by weight methyl acetate, and 0.1 to 10% byweight water.E9. The process according to any one of embodiments E1 to E8, whereinthe second absorber return stream is not returned to the separationzone.E10. The process according to any one of embodiments E1 to E9, whereinthe contacting of the absorber tower feed with the methanol or methylacetate in the second absorbent forms methyl iodide.E11. The process according to any one of embodiments E1 to E10, whereinthe conveying of the second absorber return stream comprises: conveyingthe second absorber return stream to the reactor during a transitionperiod; and following the transition period, continuing to feed thesecond absorber return stream to the reactor.E12. The process according to any one of embodiments E1 to E11, whereinthe conveying of the second absorber return stream comprises conveyingthe second absorber return stream to the reactor after the terminationof the supply of first absorbent.E13. The process according to any one of embodiments E1 to E12, whereinthe second vapor stream further comprises volatile components.E14. The process according to any one of embodiments E1 to E13, whereinthe flashing comprises condensing at least a portion of the first vaporstream to form the condensed flasher return stream and the flasher ventvapor stream comprising hydrogen iodide and directing the flasher ventvapor stream to the absorber tower.E15. The process according to any one of embodiments E1 to E14, whereinthe carbonylating comprises venting a reactor vent stream comprisinghydrogen iodide and directing at least a portion of the reactor ventstream to the absorber tower.E16. The process according to any one of embodiments E1 to E15, whereinthe carbonylating comprises condensing at least a portion of the reactorvent stream to form a condensed reactor return stream and a reactor ventvapor stream comprising hydrogen iodide and directing the reactor ventvapor stream to the absorber tower.E17. The process according to any one of embodiments E1 to E16, whereinthe second vapor stream is decanted to form a decanter vent streamcomprising hydrogen iodide and the at least a portion of the secondvapor stream comprises the decanter vent stream.E18. The process according to any one of embodiments E1 to E16, whereinthe separating further comprises condensing at least a portion of thesecond vapor stream to form a condensed light ends return stream and alight ends vent vapor stream comprising hydrogen iodide and directingthe light ends vent vapor stream to the absorber tower.E19. A process for operating an absorber tower in a carbonylationprocess, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid, optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing at least a portion of the first vapor stream toform a condensed flasher return stream and a flasher vent streamcomprising hydrogen iodide;

separating, in a light ends column, the flashed vapor stream to form asecond vapor stream comprising methyl iodide and hydrogen iodide, a sidedraw comprising purified acetic acid product, and a second liquidresidue stream;

feeding to an absorber tower at least a portion of at least one of thereactor vent stream, the flasher vent stream, and the second vaporstream to initiate a start-up period,

during the start-up period, contacting the absorber tower feed with afirst absorbent comprising acetic acid to absorb methyl iodide and toform a first absorber return stream comprising the first absorbent andthe absorbed methyl iodide;

conveying the first absorber return stream to the light ends columnand/or drying column;

transitioning from start up to steady operation during a changeoverperiod by terminating the supply of first absorbent to the absorbertower;

providing to the absorber tower a second absorbent comprising methanoland/or methyl acetate,

wherein, during at least a portion of the changeover period, aceticacid, methanol and methyl iodide are provided to the absorber tower;

forming a combined absorber return stream comprising acetic acid,methanol, and methyl iodide;

conveying the combined absorber return stream from the absorber tower tothe reactor

initiating steady state operation after the changeover period,

during steady state operation, contacting the second vapor stream with asecond absorbent comprising methanol and/or methyl acetate to absorbmethyl iodide and hydrogen iodide and to form a second absorber returnstream comprising the second absorbent and the absorbed methyl iodide;

during the steady state operation, mixing the second absorber returnstream with methanol or a reactive derivative thereof to form a mixedstream; and

conveying the mixed stream to the reactor.

E20. The process according to embodiment E19, wherein the secondabsorbent consists essentially of methanol.E21. The process according to embodiment E19 or E20, further comprisingthe step of chilling the first absorbent and/or the second absorbentprior to the contacting.E22. The process according to any one of embodiments E19 to E21, whereintermination of supply of first absorbent to the absorber tower andproviding of the second absorbent to the absorber tower occursubstantially simultaneously.E23. The process according to any one of embodiments E19 to E22, whereinat least a portion of at least two of the reactor vent stream, theflasher vent stream, and the second vapor stream are combined to formthe absorber tower feed.E24. The process according to any one of embodiments E19 to E23, whereinthe flow rate of the reactor vent stream and the second vapor stream aregreater than the combined flow rate of the remaining vent streams.E25. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid, optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide;

condensing at least a portion of the reactor vent stream to form acondensed reactor vent stream;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the condensed reactor vent stream;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide and to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide;

conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column;

decreasing the supply of first absorbent to the absorber tower;

contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide;

conveying the second extract, directly or indirectly, to the reactor.

E26. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the flasher vent stream;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide and to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide;

conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column;

decreasing the supply of first absorbent to the absorber tower;

contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide;

conveying the second extract, directly or indirectly, to the reactor.

E27. The process according to embodiment E26, wherein the flasher ventstream is condensed prior to the feeding step.E28. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second vapor stream;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide and to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide;

conveying the first extract, directly or indirectly, to the light endscolumn and/or drying column;

decreasing the supply of first absorbent to the absorber tower;

contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide;

conveying the second extract, directly or indirectly, to the reactor.

E29. The process according to embodiment E28, wherein the second vaporstream is condensed prior to the feeding step.E30. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

decanting the second vapor stream to form a light phase comprisingacetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;

separating at least a portion of the light phase and/or the heavy phasein a PRC removal system to form a PRS overhead comprising hydrogeniodide;

optionally collecting the PRS overhead in an overhead receiver andventing from the overhead receiver a PRS overhead receiver vent streamcomprising hydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb hydrogen iodide to form a first extract comprising absorbedhydrogen iodide.

E31. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid, optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

decanting the second vapor stream to form a light phase comprisingacetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide

separating at least a portion of the light phase and/or the heavy phaseto a PRC removal system to form a PRS overhead comprising hydrogeniodide;

optionally collecting the PRS overhead in an overhead receiver andventing from the overhead receiver a PRS overhead receiver vent streamcomprising hydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide to form a first extractcomprising the first absorbent and the absorbed methyl iodide andhydrogen iodide;

decreasing the supply of first absorbent to the absorber tower;

contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide.

E32. The process according to embodiment E31, further comprisingconveying the second extract, directly or indirectly, to the reactor.E33. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the flasher vent stream;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide to form a first extractcomprising the first absorbent, hydrogen iodide, and methyl iodide.

E34. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second vapor stream;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb methyl iodide and hydrogen iodide to form a first extractcomprising the first absorbent, hydrogen iodide, and methyl iodide.

E35. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

decanting the second vapor stream to form a light phase comprisingacetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;

separating at least a portion of the light phase and/or the heavy phasein a PRC removal system comprising a single column to form a PRSoverhead comprising hydrogen iodide;

optionally collecting the PRS overhead in an overhead receiver andventing from the overhead receiver a PRS overhead receiver vent streamcomprising hydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb hydrogen iodide to form a first extract comprising absorbedhydrogen iodide; and

optionally contacting the absorber tower feed with a second absorbentcomprising methanol and/or methyl acetate to absorb methyl iodide andhydrogen iodide and to form a second extract comprising the secondabsorbent and the absorbed methyl iodide.

E36. A process for producing acetic acid, the process comprising:

carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid;

flashing the crude acetic acid product, with or without heat, to form afirst vapor stream comprising acetic acid and methyl iodide and a firstliquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide;

separating, in a light ends column, the flashed first vapor stream toform a second vapor stream comprising methyl iodide and hydrogen iodide,a side draw comprising purified acetic acid product, and a second liquidresidue stream;

decanting the second vapor stream to form a light phase comprisingacetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;

separating at least a portion of the light phase and/or the heavy phasein a PRC removal system comprising a first column and a second column toobtain a second PRS overhead comprising hydrogen iodide from the secondPRC removal column;

optionally collecting the second PRS overhead in an overhead receiverand venting from the overhead receiver a second PRS overhead receivervent stream comprising hydrogen iodide;

feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second PRS overhead receiver vent stream and/or thesecond PRS overhead;

contacting the absorber tower feed with a first absorbent selected fromthe group consisting of acetic acid, methanol, and methyl acetate toabsorb hydrogen iodide to form a first extract comprising absorbedhydrogen iodide; and

optionally contacting the absorber tower feed with a second absorbentcomprising methanol and/or methyl acetate to absorb methyl iodide andhydrogen iodide and to form a second extract comprising the secondabsorbent and the absorbed methyl iodide.

E37. The process according to any one of embodiments E1 to E36, whereinthe second absorbent consists essentially of methanol.E38. The process according to any one of embodiments E1 to E37, furthercomprising the step of chilling the first absorbent or the secondabsorbent prior to the contacting.E39. The process according to any one of embodiments E1 to E38, whereinthe second vapor stream is condensed prior to the feeding step.E40. The process according to any one of embodiments E1 to E39, furthercomprising conveying the second extract, directly or indirectly, to thereactor.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference. In addition, it should beunderstood that aspects of the invention and portions of variousembodiments and various features recited below and/or in the appendedclaims may be combined or interchanged either in whole or in part. Inthe foregoing descriptions of the various embodiments, those embodimentswhich refer to another embodiment may be appropriately combined withother embodiments as will be appreciated by one of skill in the art.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

We claim:
 1. A process for producing acetic acid, the processcomprising: carbonylating, in a reactor, at least one of methanol,dimethyl ether, and methyl acetate in a reaction medium comprising ametal catalyst, methyl iodide, an iodide salt, and optionally aceticacid and a finite amount water, to form a crude acetic acid productcomprising acetic acid, optionally venting from the reactor a reactorvent stream comprising hydrogen iodide; condensing at least a portion ofthe reactor vent stream to form a condensed reactor vent stream; feedingto an absorber tower an absorber tower feed comprising at least aportion of the condensed reactor vent stream; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate to absorb methyl iodide andhydrogen iodide and to form a first extract comprising the firstabsorbent and the absorbed methyl iodide and hydrogen iodide; conveyingthe first extract, directly or indirectly, to the light ends columnand/or drying column; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor.
 2. The process of claim1, wherein the second absorbent consists essentially of methanol.
 3. Theprocess of claim 1, further comprising the step of chilling the firstabsorbent or the second absorbent prior to the contacting.
 4. Theprocess of claim 1, further comprising conveying the second extract,directly or indirectly, to the reactor.
 5. A process for producingacetic acid, the process comprising: carbonylating, in a reactor, atleast one of methanol, dimethyl ether, and methyl acetate in a reactionmedium comprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and a finite amount water, to form a crude aceticacid product comprising acetic acid; flashing the crude acetic acidproduct, with or without heat, to form a first vapor stream comprisingacetic acid and methyl iodide and a first liquid residue streamcomprising metal catalyst and halide salt, optionally condensing aportion of the first vapor stream to form a condensed flasher returnstream and a flasher vent stream comprising hydrogen iodide; feeding toan absorber tower an absorber tower feed comprising at least a portionof the flasher vent stream; contacting the absorber tower feed with afirst absorbent selected from the group consisting of acetic acid,methanol, and methyl acetate to absorb methyl iodide and hydrogen iodideand to form a first extract comprising the first absorbent and theabsorbed methyl iodide and hydrogen iodide; conveying the first extract,directly or indirectly, to the light ends column and/or drying column;decreasing the supply of first absorbent to the absorber tower;contacting the absorber tower feed with a second absorbent comprisingmethanol and/or methyl acetate to absorb methyl iodide and hydrogeniodide and to form a second extract comprising the second absorbent andthe absorbed methyl iodide; conveying the second extract, directly orindirectly, to the reactor.
 6. The process of claim 5, wherein thesecond absorbent consists essentially of methanol.
 7. The process ofclaim 5, further comprising the step of chilling the first absorbent orthe second absorbent prior to the contacting.
 8. The process of claim 5,wherein the flasher vent stream is condensed prior to the feeding step.9. The process of claim 5, further comprising conveying the secondextract, directly or indirectly, to the reactor.
 10. A process forproducing acetic acid, the process comprising: carbonylating, in areactor, at least one of methanol, dimethyl ether, and methyl acetate ina reaction medium comprising a metal catalyst, methyl iodide, an iodidesalt, and optionally acetic acid and a finite amount water, to form acrude acetic acid product comprising acetic acid; flashing the crudeacetic acid product, with or without heat, to form a first vapor streamcomprising acetic acid and methyl iodide and a first liquid residuestream comprising metal catalyst and halide salt; separating, in a lightends column, the flashed first vapor stream to form a second vaporstream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; feeding to an absorber tower an absorber tower feed comprisingat least a portion of the second vapor stream; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate to absorb methyl iodide andhydrogen iodide and to form a first extract comprising the firstabsorbent and the absorbed methyl iodide and hydrogen iodide; conveyingthe first extract, directly or indirectly, to the light ends columnand/or drying column; decreasing the supply of first absorbent to theabsorber tower; contacting the absorber tower feed with a secondabsorbent comprising methanol and/or methyl acetate to absorb methyliodide and hydrogen iodide and to form a second extract comprising thesecond absorbent and the absorbed methyl iodide; conveying the secondextract, directly or indirectly, to the reactor.
 11. The process ofclaim 10, wherein the second absorbent consists essentially of methanol.12. The process of claim 10, further comprising the step of chilling thefirst absorbent or the second absorbent prior to the contacting.
 13. Theprocess of claim 10, wherein the second vapor stream is condensed priorto the feeding step.
 14. The process of claim 10, further comprisingconveying the second extract, directly or indirectly, to the reactor.15. A process for producing acetic acid, the process comprising:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid, optionally venting from the reactor a reactor vent streamcomprising hydrogen iodide; flashing the crude acetic acid product, withor without heat, to form a first vapor stream comprising acetic acid andmethyl iodide and a first liquid residue stream comprising metalcatalyst and halide salt, optionally condensing a portion of the firstvapor stream to form a condensed flasher return stream and a flashervent stream comprising hydrogen iodide; separating, in a light endscolumn, the flashed first vapor stream to form a second vapor streamcomprising methyl iodide and hydrogen iodide, a side draw comprisingpurified acetic acid product, and a second liquid residue stream;decanting the second vapor stream to form a light phase comprisingacetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodideseparating at least a portion of the light phase and/or the heavy phaseto a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiverand venting from the overhead receiver a PRS overhead receiver ventstream comprising hydrogen iodide; feeding to an absorber tower anabsorber tower feed comprising at least a portion of the PRS overheadreceiver vent stream and/or the PRS overhead; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate to absorb methyl iodide andhydrogen iodide to form a first extract comprising the first absorbentand the absorbed methyl iodide and hydrogen iodide; decreasing thesupply of first absorbent to the absorber tower; contacting the absorbertower feed with a second absorbent comprising methanol and/or methylacetate to absorb methyl iodide and hydrogen iodide and to form a secondextract comprising the second absorbent and the absorbed methyl iodide.16. The process of claim 15, wherein the second absorbent consistsessentially of methanol.
 17. The process of claim 15, further comprisingthe step of chilling the first absorbent or the second absorbent priorto the contacting.
 18. The process of claim 15, wherein the second vaporstream is condensed prior to the feeding step.
 19. The process of claim15, further comprising conveying the second extract, directly orindirectly, to the reactor.
 20. A process for producing acetic acid, theprocess comprising: carbonylating, in a reactor, at least one ofmethanol, dimethyl ether, and methyl acetate in a reaction mediumcomprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and a finite amount water, to form a crude aceticacid product comprising acetic acid; flashing the crude acetic acidproduct, with or without heat, to form a first vapor stream comprisingacetic acid and methyl iodide and a first liquid residue streamcomprising metal catalyst and halide salt, optionally condensing aportion of the first vapor stream to form a condensed flasher returnstream and a flasher vent stream comprising hydrogen iodide; feeding toan absorber tower an absorber tower feed comprising at least a portionof the flasher vent stream; contacting the absorber tower feed with afirst absorbent selected from the group consisting of acetic acid,methanol, and methyl acetate to absorb methyl iodide and hydrogen iodideto form a first extract comprising the first absorbent, hydrogen iodide,and methyl iodide.
 21. The process of claim 20, wherein the firstabsorbent is acetic acid.
 22. A process for producing acetic acid, theprocess comprising: carbonylating, in a reactor, at least one ofmethanol, dimethyl ether, and methyl acetate in a reaction mediumcomprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and a finite amount water, to form a crude aceticacid product comprising acetic acid; flashing the crude acetic acidproduct, with or without heat, to form a first vapor stream comprisingacetic acid and methyl iodide and a first liquid residue streamcomprising metal catalyst and halide salt; separating, in a light endscolumn, the flashed first vapor stream to form a second vapor streamcomprising methyl iodide and hydrogen iodide, a side draw comprisingpurified acetic acid product, and a second liquid residue stream;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second vapor stream; contacting the absorber tower feedwith a first absorbent selected from the group consisting of aceticacid, methanol, and methyl acetate to absorb methyl iodide and hydrogeniodide to form a first extract comprising the first absorbent, hydrogeniodide, and methyl iodide.
 23. The process of claim 22, wherein thefirst absorbent is acetic acid.
 24. A process for producing acetic acid,the process comprising: carbonylating, in a reactor, at least one ofmethanol, dimethyl ether, and methyl acetate in a reaction mediumcomprising a metal catalyst, methyl iodide, an iodide salt, andoptionally acetic acid and a finite amount water, to form a crude aceticacid product comprising acetic acid; flashing the crude acetic acidproduct, with or without heat, to form a first vapor stream comprisingacetic acid and methyl iodide and a first liquid residue streamcomprising metal catalyst and halide salt, optionally condensing aportion of the first vapor stream to form a condensed flasher returnstream and a flasher vent stream comprising hydrogen iodide; separating,in a light ends column, the flashed first vapor stream to form a secondvapor stream comprising methyl iodide and hydrogen iodide, a side drawcomprising purified acetic acid product, and a second liquid residuestream; decanting the second vapor stream to form a light phasecomprising acetaldehyde and hydrogen iodide and a heavy phase comprisingacetaldehyde, methyl iodide, methyl acetate, and hydrogen iodide;separating at least a portion of the light phase and/or the heavy phasein a PRC removal system to form a PRS overhead comprising hydrogeniodide; optionally collecting the PRS overhead in an overhead receiverand venting from the overhead receiver a PRS overhead receiver ventstream comprising hydrogen iodide; feeding to an absorber tower anabsorber tower feed comprising at least a portion of the PRS overheadreceiver vent stream and/or the PRS overhead; contacting the absorbertower feed with a first absorbent selected from the group consisting ofacetic acid, methanol, and methyl acetate to absorb hydrogen iodide toform a first extract comprising absorbed hydrogen iodide.
 25. Theprocess of claim 20, wherein the first absorbent is acetic acid.
 26. Aprocess for producing acetic acid, the process comprising:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; separating, in a light ends column, the flashed firstvapor stream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a single column toform a PRS overhead comprising hydrogen iodide; optionally collectingthe PRS overhead in an overhead receiver and venting from the overheadreceiver a PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the PRS overhead receiver vent stream and/or the PRSoverhead; contacting the absorber tower feed with a first absorbentselected from the group consisting of acetic acid, methanol, and methylacetate to absorb hydrogen iodide to form a first extract comprisingabsorbed hydrogen iodide; and optionally contacting the absorber towerfeed with a second absorbent comprising methanol and/or methyl acetateto absorb methyl iodide and hydrogen iodide and to form a second extractcomprising the second absorbent and the absorbed methyl iodide.
 27. Theprocess of claim 26, wherein the first absorbent is acetic acid.
 28. Aprocess for producing acetic acid, the process comprising:carbonylating, in a reactor, at least one of methanol, dimethyl ether,and methyl acetate in a reaction medium comprising a metal catalyst,methyl iodide, an iodide salt, and optionally acetic acid and a finiteamount water, to form a crude acetic acid product comprising aceticacid; flashing the crude acetic acid product, with or without heat, toform a first vapor stream comprising acetic acid and methyl iodide and afirst liquid residue stream comprising metal catalyst and halide salt,optionally condensing a portion of the first vapor stream to form acondensed flasher return stream and a flasher vent stream comprisinghydrogen iodide; separating, in a light ends column, the flashed firstvapor stream to form a second vapor stream comprising methyl iodide andhydrogen iodide, a side draw comprising purified acetic acid product,and a second liquid residue stream; decanting the second vapor stream toform a light phase comprising acetaldehyde and hydrogen iodide and aheavy phase comprising acetaldehyde, methyl iodide, methyl acetate, andhydrogen iodide; separating at least a portion of the light phase and/orthe heavy phase in a PRC removal system comprising a first column and asecond column to obtain a second PRS overhead comprising hydrogen iodidefrom the second PRC removal column; optionally collecting the second PRSoverhead in an overhead receiver and venting from the overhead receivera second PRS overhead receiver vent stream comprising hydrogen iodide;feeding to an absorber tower an absorber tower feed comprising at leasta portion of the second PRS overhead receiver vent stream and/or thesecond PRS overhead; contacting the absorber tower feed with a firstabsorbent selected from the group consisting of acetic acid, methanol,and methyl acetate to absorb hydrogen iodide to form a first extractcomprising absorbed hydrogen iodide; and optionally contacting theabsorber tower feed with a second absorbent comprising methanol and/ormethyl acetate to absorb methyl iodide and hydrogen iodide and to form asecond extract comprising the second absorbent and the absorbed methyliodide.
 29. The process of claim 28, wherein the first absorbent isacetic acid.